A new state, GTK_STATE_FLAG_CHECKED, has been added for checked states of radio and check buttons and menuitems. Applications that are using GTK+ styles without widgets will need adjustments.
Adwaita is now the default theme on all platforms.
The icon theme code has become a little pickier about sizes and is not automatically scaling icons beyond the limits defined in the icon theme unless explicitly asked to do so with GTK_ICON_LOOKUP_FORCE_SIZE.
GTK+ now includes an interactive debugger which can be activated with the keyboard shortcuts Ctrl-Shift-d or Ctrl-Shift-i. If these shortcuts interfere with application keybindings, they can be disabled with the setting org.gtk.Settings.Debug.enable-inspector-keybinding.
Most widgets have been ported to use the new gesture framework internally for event handling. Traditional event handlers in derived widgets are still being called.
Using GTK+ under X11 without the X Render extension has been observed to be problematic. This combination is using code paths in cairo and graphics drivers which are rarely tested and likely buggy.
GtkTextView is now using a pixel-cache internally, and is drawing a background underneath the text. This can cause problems for applications which assumed that they could draw things below and above the text by chaining up in the ::draw implementation of their GtkTextView subclass. As a short-term workaround, you can make the application apply a custom theme to the text view with a transparent background. For a proper fix, use the new ::draw_layer vfunc.