I think it's fair to start with an app that I use most and that is most likely Evolution (especially its e-mail component). The UI is reworked and I'd say it both looks better and is more usable, it's almost pain to return to F12 Evolution's interface. On the flip side, IMAP backend has also been rewritten (or so I've heard) and it hits a dead end here and there and because of that, using Evolution isn't very smooth and joyful. So in short, with future releases there is high probability Evolution will be significantly better that what's in F12, but currently it's one of the pieces that blocks me from advancing to F13. Plus, I forgot to mention, Evolution lost the option to keep gpg passphrase stored during session.
Intel Video Driver
Well, this one's a little bit of pain. I've grown quite used to F12 which has been damn stable so fast, so X or even the whole notebook frozen was a big surprise. Seems the frozenness of whole system have been already fixed, but last time I were in updated F13 (about a week ago), I still managed somehow to endup with half-frozen X — I move with mouse, switch to tty, kill apps (without any graphical response though) or kill X completely. But only full reboot fixed it for good.
Another UI change. Generally it seems cleaner, but what irritates me is that I cannot keep the entry-like input for paths — I seem to be able to access it only via ctrl-l and after entering the path, it reverts back to the button mode. No good, no good.
This where the most goodness (from my point of view) went, albeit not exactly as I'd prefer it. Thanks to hard work the guys over at webkit and epiphany, epiphany now can store passwords, but as epiphany does not have sensible session saving (why the hell do I need to kill it to save session?) [and for some other reasons] I use midori, which however it does not seem to know how to save passwords (in web forms, not in web auth pop-ups). The rest of the goodness (I know about) is same for both browsers — html5 youtube flawless support (only fullscreen seems to be missing), html5 video tag finally has handler, sites (like anidb) which serve pages in compressed format are now supported and java applet (I've tried only the openjdk one) is almost working.
In one word it behaves strange. Sometimes the keyring that should get unlocked at startups refuses to unlock, sometimes I get asked for new password for 'default' keyring, which however never gets created… This is incidentally also the last thing that shows when trying to start up gajim. The good thing is that thanks to this I started using emapthy with a chat theme from one of my favourite anime durarara >_<
Well, there are still some hick-ups that keep me from transferring from the probably best fedora release so far (F12) to F13 to feel the new awesomeness, and one thing to keep in my mind is: don't be lazy and finally fill the bugs you've encountered!