set-fill-column no longer requires a prefix argument. So where I used to have to do C-u 100 RET C-x f I can now just do C-x f RET 100 RET which is much more semantically pleasing.

Tab completion now fills in blanks where it can unambiguously. So for example, C-x C-f nesc-in TAB will complete to nesc-workshop-invitation.txt.

A slightly weird one is that, for wrapped lines, next-line (C-n or pressing the down arrow) instead of moving the cursor to the next line of the buffer (as it used to) now moves the cursor to the next line on the screen, even if it's within the same line of the buffer. Similarly for previous-line. The result is that Emacs now behaves a bit more like GUI text editors (such as Kate and Gedit). It's quite easy to get used to but was a bit of shock when I first noticed it.

Anti-aliased fonts in Gtk Emacs are very lovely indeed :-)