Most obviously - things that would no longer work in 3, that used to work in 2 - if that was not a problem, then it would've been an issue to switch (and the performance is not worse than 2).
But I guess it's not that easy. Also the "C" api must be backward-compatible.
If You skim through Armin Ronacher blog [1] and you will find out pretty soon :) Also there is a Q&A for Python 2 vs 3 by Nick Coghlan. [2]
Basically Python 3 makes it much-much harder working with legacy POSIX system and web frameworks where you need to do a bunch of unicode <--> bytes encoding dance seemingly unnecessary.
