Just an update about problems with Yosemite and Photoshop Elements.
To summarize, for now it appears that PSE 9 and lower work fine in 10.10, as does PSE 13. PSE 10 is also okay, unless you bought it from the Mac App Store, in which case it has a disconcerting tendency to switch languages to German on updating to 10.10.
As far as I know there’s no real solution for this, at least not yet. Something similar happened after an OS X update back when PSE 10 was the current version of PSE, and this was the suggestion, then, but I haven’t heard of it working now, since OS X has changed a lot since then:
PSE 11 has some problems in Yosemite, but the biggest problems are with PSE 12. First of all, there are problems if you use a trackpad, and Adobe is recommending using a mouse as a workaround. But even without a trackpad some people are having problems, especially with the Move tool.
Adobe has announced that they are working with Apple to try to resolve the PSE 11 and 12 problems in an upcoming update (by which I’m presuming they mean an OS X update, but they don’t specify). They’ve also posted plugin that is supposed to help with the problem for now. See Jeffrey Tranberry’s post in this thread for more information and a download link:
EDIT (Oops, the discount link is gone today–they must have removed it shortly after posting the plugin.)