The list of changes/tl;dr:

Weather support
Redesigned GUI
Back button in help GUI
Windows updater ('Update' button on the main client screen)
Many new background musics
Refresh server buttons on the main client screen
Direction markers in region map
Ability to rebind shift/ctrl/alt/etc keys
Can now use keyboard modifiers for keybindings
Drop/get keys no longer get stuck on multiple quantities when holding down the key
Widgets can no longer be moved off-screen by default
Fixed name and HP percent of multi-part monster objects not appearing properly in corners of the client map.
Readied objects such as ammo and throwing items are now preserved across sessions
Player doll now shows your archery damage
Some client markup bugfixes
Massive book GUI performance optimization
Better looking fonts on Windows
The map is now a widget, and the right-click menu of it can be accessed by right-clicking one of the map squares and holding down right mouse button for about half a second (until the menu pops up). If the button is released in less than about half a second, it will trigger a target enemy/friend command, like the old behavior
Better console commands tab auto-completion; instead of showing all matching commands when there is more than one, it will cycle through them using the tab key (irssi-like behavior, type / in the console and press tab; you can then press tab again to switch to the next matching command, and partial commands are also supported)
Better links color in book GUI
Widget (sub-)menus will no longer go over the maximum screen size
Recently killed corpses are now highlighted in red if you have infravision
Clipboard support: it is now possible to paste text into console using ctrl+v, and select text in text windows and use ctrl+c to copy it, and paste it into another application on your desktop
Widgets that support resizing (such as text windows) can now be resized by dragging either of their sides, or one of their corners (to resize both vertically and horizontally at once)
Text windows font size change has moved from client settings to the text windows right-click menu
Text windows right-click menu now has a 'clear' action, which clears the text in the text window
In order to move a widget now, you have to right-click it, and press the 'Move' entry once; now, the widget will stick to the mouse cursor, instead of requiring you to continue holding the mouse button. In order to drop the widget, press any of the mouse buttons again.
Widgets will now automatically snap to nearby widgets when being moved, allowing for easy and pixel-perfect alignment
The client can now run even if sound system is not available
Character's password can now be changed using the ESC menu, and clicking the Password button
Various other little tweaks and bugfixes