I should probably give Zotero another try. At the time I started using Mendeley, it was really the best at automatically importing metadata for pdfs. And I don't think Zotero does the watch folder thing.

One thing I do like about Mendeley is that it's content to leave my pdfs alone in the filesystem (and I can rely on Dropbox for syncing those), whereas Zotero seems to want to import everything and manage the files itself.