I guess nothing.
I like 2Fa, I don't know about in 3.3 but I'd like to see it. Why not? Users like being able to protect themselves against their own mistakes.
I guess ajaxifying user interaction as reasonably as possible would be another good thing. Quick replies, editing posts, chat like pms...
A third thing? I don't really know. Things I really do need would be best served as extensions at this point, or just more ajax suggestions.
Oh! I know, this is a good one. How about having the extension db tied into the ACP like wordpress has their extensions, including one (or two click, installation and activation) installs?
Is there an extension that makes a pop up login box? Would be great for mobile especially.
Something I've been thinking about is how a lot of people who use base phpbb don't know about extensions or don't think they can provide what they can. They can change your forum so much it's ridiculous. I don't have any solutions to this but it is something I've been thinking about...
I am a web developer/administrator, specializing in forums. If you have work you need done or are too lazy to do, pm me!