Right now Dropbox's changelog is full of entries like "More fixes for Finder on Mac OSX 10.10". I didn't notice your issues, but Dropbox kept hanging since the Yosemite upgrade.
I just updated to the latest version from Dropbox website, which got me release 2.10.41, and now everything you mention works fine (for now). But from the changelog link, you can download later stable releases which might work better.
Let me speculate on the reason: while before Yosemite Dropbox used a huge hack to extend Finder, now Apple introduced clean support for Finder Extensions, so the Dropbox integration with Finder was probably rewritten. In theory, this should make things more stable; apparently, though, the new code is not good enough yet.