I've noticed a new issue lately with AOL chat rooms in Messages. iMovie (latest version) doesn't work in Yosemite without a terminal hackaround. Lots of little graphic glitches in programs that do run properly but aren't updated for "dark" mode or just Yosemite overall. Had to get special versions from the developers of Cyberduck and Forklift before either one would work properly, and Locko is still non-functional (new update on the way). 1Password sorta works, sorta doesn't in the browser plug-in.
In short, lots of little things, no show-stoppers unless you need FTP programs (Filezilla works, but Cheeses Cripes is it ever ugly compared to CyberDuck/Forklift!!). I was quite astonished to note that Photoshop CS3 (yes, that's right CS3 from 2007) works, though of course some plug-ins do not.
It's interesting to open an "old" program in Yosemite and note how many elements rely on Apple APIs for appearance (which you can tell because they have changed in Yosemite). So far, overall, most things work as expected but yes, it's not quite ready for prime time and yes, it's A BETA and should not be used on your moneymaker machines unless you like getting caught short when your "assumption" that program X will work turns out to be not the case!