there are a lot of "warning: foo defined in blah.h is deprecated" type warnings that scroll by as one compiles kde4. totally expected, but we'll eventually want to do something about it.
makes for a great junior job, really. if you'd like to get a feel for what kde4 is like but want to start in the safer end of the pool, think about taking on these deprecated warnings. virtually all of them are listed in kdelibs/KDE4PORTING.html (though a few bits are also here).
however, every so often i come across an api change that isn't well documented. there is no excuse for this. the kaction changes, for instance, aren't documented anywhere that i could find (and now that i've said that, some one will point me to the exact location in svn within the next 30 seconds).
i also saw a deprecation warning for kdelibs/kio/bookmarks/kbookmarkimporter_ns.h. sure enough, everything is marked as deprecated ... but there are absolutely no notes on what to replace those calls with. i suppose i could become overly intimate with the rest of the bookmark api, but then i'd rather just have some nice clear documentation as well.
and here's where i hand off to ade, my favourite documentarian. ;-P