I've been using Mac OS X 10.10 [Yosemite] for about a week now, and it has good features and bad.
First, the good: In OS X 10.9 [Mavericks], the Preview application allowed me to annotate my Poser renderings and other pictures with speech balloons. Yosemite improves on that by allowing speech balloons to be resized and their tails repositioned, just as in Comic Life. I'm STILL waiting for Digimage Arts to finish the new version of Color-It! for Intel Macs. I use Paintbrush, which is open source, but it just doesn't have useful features like smudging, non-rectangular selection and pattern fills.
Now, the bad: I've always cleared my history menu before quitting a Safari session. Now when I use Safari under Yosemite, I have to go through a logon screen every time I visit a Web site that requires a username and password. At least Safari's AutoFill completes the forms automatically, but I still consider this behavior awkward and not befitting Apple.