For end-users, GTK+ 3.10 is significant in that it should have near-complete HiDPI support -- the tool-kit playing nicely with retina-like displays. There's been a lot of work in recent months by GNOME on their HiDPI support throughout the desktop. The HiDPI blocker right now is just waiting on a new Cairo release.
GTK+ Clutter integration is ongoing for implementing the Intel-backed tool-kit inside GTK. However, not all of the work may bear fruit until GTK+ 3.12. Some other items postponed to the GTK+ 3.11 development cycle for GTK+ 3.12 include GProperty and pushing GtkClipboard into GDK.
While there's been initial Wayland support going back to early GTK+3, some of the GTK+ 3.10 work items at the moment include GtkClipboard support for Wayland as well as menu positioning with an intent-based API.
More details on the GTK+ plans that were devised at GUADEC 2013 can be found on the GNOME Wiki and via this blog post.