Leaflet 0.4 contains several API improvements that allow simpler, jQuery-like syntax ([example](https://gist.github.com/3038879)) while being backwards compatible with the previous approach (so you can use both styles):
* Improved most methods and options to accept `LatLng`, `LatLngBounds`, `Point` and `Bounds` values in an array form (e.g. `map.panTo([lat, lng])` will be the same as `map.panTo(new L.LatLng(lat, lng))`)
* Added `addTo` method to all layer classes, e.g. `marker.addTo(map)` is equivalent to `map.addLayer(marker)`
* Added factory methods to most classes to be able to write code without `new` keyword, named the same as classes but starting with a lowercase letter, e.g. `L.map('map')` is the same as `new L.Map('map')`
* Improved zooming so that you don't get a blank map when you zoom in or out twice quickly (by [@danzel](https://github.com/danzel)). [#7](https://github.com/CloudMade/Leaflet/issues/7) [#729](https://github.com/CloudMade/Leaflet/pull/729)
* Drag-panning now works even when there are markers in the starting point (helps on maps with lots of markers). [#506](https://github.com/CloudMade/Leaflet/issues/506)
* Added `TileLayer``detectRetina` option (`false` by default) that makes tiles show in a higher resolution on iOS retina displays (by [@Mithgol](https://github.com/Mithgol)). [#586](https://github.com/CloudMade/Leaflet/pull/586)
GeoJSON API was improved to be simpler and more flexible ([example](https://gist.github.com/3062900)). The changes are not backwards-compatible, so be sure to update your old code.
* Added `style` option for styling vector layers, passed either as an object or as a function (to style vector layers according to GeoJSON properties).
* Added `filter` option to leave out features that don't correspond to a certain criteria (e.g. based on properties).
* Added `onEachFeature` option to execute certain code on each feature layer based on its properties (binding popups, etc).
* Changed `pointToLayer` function signature to provide `geojson` in addition to `latlng` when creating point features for more flexibility.
Icon API was improved to be more flexible, but one of the changes is backwards-incompatible: you now need to pass different icon properties (like `iconUrl`) inside an options object ([example](https://gist.github.com/3076084)).
* Converted `Icon` properties to options, changed constructor signature to `Icon(options)`.
* Added `Icon``className` option to assign a custom class to an icon.
* Added `Icon``shadowOffset` option to set the position of shadow relative to the icon.
* Made all `Icon` options except `iconUrl` optional (if not specified, they'll be chosen automatically or implemented using CSS). Anchor is centered by default (if size is specified), and otherwise can be set through CSS using negative margins.
* Improved `on` and `off` methods to also accept `(eventHash[, context])`, as well as multiple space-separated events (by [@Guiswa](https://github.com/Guiswa)). [#770](https://github.com/CloudMade/Leaflet/pull/770)
* Improved `off` to remove all listeners of the event if no function was specified (by [@Guiswa](https://github.com/Guiswa)). [#770](https://github.com/CloudMade/Leaflet/pull/770) [#691](https://github.com/CloudMade/Leaflet/issues/691)
* Added `on` and `off` aliases for `DomEvent``addListener` and `removeListener` methods.
* Added `containerPoint` property to `MouseEvent`. [#413](https://github.com/CloudMade/Leaflet/issues/413)
* Added `LatLngBounds``pad` method that returns bounds extended by a percentage (by [@jacobtoye](https://github.com/jacobtoye)). [#492](https://github.com/CloudMade/Leaflet/pull/492)
* Added `L_NO_TOUCH` global variable switch (set it before Leaflet inclusion) which disables touch detection, helpful for desktop apps built using QT. [#572](https://github.com/CloudMade/Leaflet/issues/572)
* Fixed a bug where the map was zooming incorrectly inside a `position: fixed` container (by [@chx007](https://github.com/chx007)). [#602](https://github.com/CloudMade/Leaflet/pull/602)
* Fixed a bug where scaled tiles weren't cleared up after zoom in some cases (by [@cfis](https://github.com/cfis)) [#683](https://github.com/CloudMade/Leaflet/pull/683)
* Fixed a bug where `TileLayer.WMS` wouldn't take `insertAtTheBottom` option into account (by [@bmcbride](https://github.com/bmcbride)). [#478](https://github.com/CloudMade/Leaflet/pull/478)
* Fixed a bug where marker click event would stop working if you dragged it and then disabled dragging. [#434](https://github.com/CloudMade/Leaflet/issues/434)
* Fixed a bug where vector layer `setStyle({stroke: false})` wouldn't remove stroke and the same for fill. [#441](https://github.com/CloudMade/Leaflet/issues/441)
* Fixed a bug where `TileLayer``load` event wasn't fired if some tile didn't load (by [@cfis](https://github.com/cfis)) [#682](https://github.com/CloudMade/Leaflet/pull/682)
* Fixed error when calling `GeoJSON``clearLayer` (by [@runderwood](https://github.com/runderwood)). [#617](https://github.com/CloudMade/Leaflet/pull/617)
* Fixed a bug where polygons/polylines sometimes throwed an error when making them editable manually (by [@cfis](https://github.com/cfis)). [#669](https://github.com/CloudMade/Leaflet/pull/669)
* Fixed a bug where `Control``setPosition` wasn't always working correctly (by [@ericmmartinez](https://github.com/ericmmartinez)). [#657](https://github.com/CloudMade/Leaflet/pull/657)
* Fixed a bug with `Util.bind` sometimes losing arguments (by [@johtso](https://github.com/johtso)). [#588](https://github.com/CloudMade/Leaflet/pull/588)
* Fixed a bug where `TileLayer``load` event was firing only once (by [@lapinos03](https://github.com/lapinos03) and [shintonik](https://github.com/shintonik)). [#742](https://github.com/CloudMade/Leaflet/pull/742) [#177](https://github.com/CloudMade/Leaflet/issues/177)
* Fixed a bug where `FeatureGroup` popup events where not cleaned up after removing a layer from it (by [@danzel](https://github.com/danzel)). [#775](https://github.com/CloudMade/Leaflet/pull/775)
* Fixed a bug where clickable paths on IE9 didn't have a hand cursor (by [naehrstoff](https://github.com/naehrstoff)). [#671](https://github.com/CloudMade/Leaflet/pull/671)
* Fixed a bug in IE with disappearing icons when changing opacity (by [@tagliala](https://github.com/tagliala) and [DamonOehlman](https://github.com/DamonOehlman)). [#667](https://github.com/CloudMade/Leaflet/pull/667) [#600](https://github.com/CloudMade/Leaflet/pull/600)
* Fixed a bug that caused popups to be empty in IE when passing a DOM node as the content (by [@nrenner](https://github.com/nrenner)). [#472](https://github.com/CloudMade/Leaflet/pull/472)
* Fixed a bug that could cause false `mousemove` events on click in Chrome (by [@stsydow](https://github.com/stsydow)). [#757](https://github.com/CloudMade/Leaflet/pull/757)
* Fixed a bug in IE6-8 where adding fill or stroke on vector layers after initialization with `setStyle` would break the map. [#641](https://github.com/CloudMade/Leaflet/issues/641)
* Fixed a bug that sometimes caused map to disappear completely after zoom on iOS (by [@fr1n63](https://github.com/fr1n63)). [#580](https://github.com/CloudMade/Leaflet/issues/580) [#777](https://github.com/CloudMade/Leaflet/pull/777)
* Fixed a bug where opening the layers control would propagate a click to the map (by [@jacobtoye](https://github.com/jacobtoye)). [#638](https://github.com/CloudMade/Leaflet/pull/638)
* Added **Canvas backend** for vector layers (polylines, polygons, circles). This enables vector support on Android <3,anditcanalsobeoptionallypreferredoverSVGforaperformancegaininsomecases.Thanksto [@florianf](https://github.com/florianf) forabigpartofthiswork.
* When panning to another copy of the world (that's infinite horizontally), map overlays now jump to corresponding positions. [#273](https://github.com/CloudMade/Leaflet/issues/273)
* Limited maximum zoom change on a single mouse wheel movement (so you won't zoom across the whole zoom range in one scroll). [#149](https://github.com/CloudMade/Leaflet/issues/149)
* Added `Map``zoomstart` event (thanks to [@Fabiz](https://github.com/Fabiz)). [#377](https://github.com/CloudMade/Leaflet/pull/377)
* Improved `Map``locate` method, added ability to watch location continuously and more options. [#212](https://github.com/CloudMade/Leaflet/issues/212)
* Added second argument `inside` to `Map``getBoundsZoom` method that allows you to get appropriate zoom for the view to fit *inside* the given bounds.
* Added `urlParams` third optional argument to `TileLayer` constructor for convenience: an object with properties that will be evaluated in the URL template.
* Added `Polyline``closestLayerPoint` method that's can be useful for interaction features (by [@anru](https://github.com/anru)). [#186](https://github.com/CloudMade/Leaflet/pull/186)
* Added `minWidth` option to `Popup` (by [@marphi](https://github.com/marphi)). [#214](https://github.com/CloudMade/Leaflet/pull/214)
* Improved `LatLng` constructor to be more tolerant (and throw descriptive error if latitude or longitude can't be interpreted as a number). [#136](https://github.com/CloudMade/Leaflet/issues/136)
* Made `LatLngBounds``extend` method to accept other `LatLngBounds` in addition to `LatLng` (by [@JasonSanford](https://github.com/JasonSanford)). [#553](https://github.com/CloudMade/Leaflet/pull/553)
* Improved browser-specific code to rely more on feature detection rather than user agent string.
* Improved superclass access mechanism to work with inheritance chains of 3 or more classes; now you should use `Klass.superclass` instead of `this.superclass` (by [@anru](https://github.com/anru)). [#179](https://github.com/CloudMade/Leaflet/pull/179)
* Fixed a bug where `Circle` was rendered with incorrect radius (didn't take projection exagerration into account). [#331](https://github.com/CloudMade/Leaflet/issues/331)
* Fixed a bug where map panning would stuck forever after releasing the mouse over an iframe or a flash object (thanks to [@sten82](https://github.com/sten82)). [#297](https://github.com/CloudMade/Leaflet/pull/297) [#64](https://github.com/CloudMade/Leaflet/issues/64)
* Fixed a bug where mouse wheel zoom worked incorrectly if map is inside scrolled container (partially by [@chrillo](https://github.com/chrillo)). [#206](https://github.com/CloudMade/Leaflet/issues/206)
* Fixed a bug where `Circle` was rendered with incorrect radius (didn't take projection exaggeration into account). [#331](https://github.com/CloudMade/Leaflet/issues/331)
* Fixed a bug where `Marker``setIcon` was not working properly (by [@marphi](https://github.com/marphi)). [#218](https://github.com/CloudMade/Leaflet/pull/218) [#311](https://github.com/CloudMade/Leaflet/issues/311)
* Fixed a bug where `Marker``setLatLng` was not working if it's set before adding the marker to a map. [#222](https://github.com/CloudMade/Leaflet/issues/222)
* Fixed a bug where marker popup would not move on `Marker``setLatLng` (by [@tjarratt](https://github.com/tjarratt)). [#272](https://github.com/CloudMade/Leaflet/pull/272)
* Fixed a bug that caused en error when dragging marker with icon without shadow (by [@anru](https://github.com/anru)). [#178](https://github.com/CloudMade/Leaflet/issues/178)
* Fixed a bug where drag event fired before the actual movement of layer (by [@anru](https://github.com/anru)). [#197](https://github.com/CloudMade/Leaflet/pull/197)
* Fixed a bug where `TileLayer``load` event wouldn't fire in some edge cases (by [@dravnic](https://github.com/dravnic)).
* Fixed a bug related to clearing background tiles after zooming (by [@neno-giscloud](https://github.com/neno-giscloud) & [@dravnic](https://github.com/dravnic)).
* Fixed a bug related to cleaning up after removing tile layers (by [@dravnic](https://github.com/dravnic)). [#276](https://github.com/CloudMade/Leaflet/pull/276)
* Fixed a bug in Elliptical Mercator formula that affeted `EPSG:3395` CRS (by [@Savvkin](https://github.com/Savvkin)). [#358](https://github.com/CloudMade/Leaflet/pull/358)
* Fixed occasional crashes on Mac Safari (thanks to [@lapinos03](https://github.com/lapinos03)). [#191](https://github.com/CloudMade/Leaflet/issues/191)
* Fixed a bug where resizing the map would sometimes make it blurry on WebKit (by [@mortenbekditlevsen](https://github.com/mortenbekditlevsen)). [#453](https://github.com/CloudMade/Leaflet/pull/453)
* Fixed a bug that caused performance drop and high CPU usage when calling `setView` or `panTo` to the current center. [#231](https://github.com/CloudMade/Leaflet/issues/231)
* Fixed crash on Android 3+ when panning or zooming (by [@florian](https://github.com/florianf)). [#137](https://github.com/CloudMade/Leaflet/issues/137)
* Added different **projections** support, having `EPSG:3857`, `EPSG:4326` and `EPSG:3395` out of the box (through `crs` option in `Map`). Thanks to [@Miroff](https://github.com/Miroff) & [@Komzpa](https://github.com/Komzpa) for great advice and explanation regarding this.
* Improved panning performance in Chrome and FF considerably with the help of `requestAnimationFrame`. [#130](https://github.com/CloudMade/Leaflet/issues/130)
* Fixed a bug where zooming is broken if the map contains a polygon and you zoom to an area where it's not visible. [#47](https://github.com/CloudMade/Leaflet/issues/47)
* Fixed a bug where marker that was added, removed and then added again would not appear on the map. [#66](https://github.com/CloudMade/Leaflet/issues/66)
* Disabled zoom animation on Android by default because it's buggy on some devices (will be enabled back when it's stable enough). [#32](https://github.com/CloudMade/Leaflet/issues/32)
* Fixed a bug where map would occasionally break while multi-touch-zooming on iOS. [#32](https://github.com/CloudMade/Leaflet/issues/32)
* Fixed potentional memory leak on WebKit when removing tiles, thanks to [@Scalar4eg](https://github.com/Scalar4eg). [#107](https://github.com/CloudMade/Leaflet/issues/107)