I had a question removed because they said it was to start a discussion, not short answers. The question was about the annular eclipse we had in May, really I just wanted an excuse to post the really cool picture I took of the shadows it made.
In another I put a link to one of my hubs, it was a question about other people's poor performing hubs with invitation to link to them, apparently this is not allowed. That question was never even published.
These are the only questions I've ever had disallowed. I'm sure there are other reasons for questions being disallowed or removed.