I'm one of those people who reads User Agreements, despite the fine print and legal language. In the past, I have cautioned against posting on Zuda and Drunk Duck, and I have expressed mild concern about posting on ComicsGenesis.
Zuda and Drunk Duck are quite clear about claiming certain rights to material posted on their sites. ComicSpace imposes certain responsibilities in the contract, like linking back. Online Comics also requires linking back, but it appears as part of the enrollment and not, as far as I can see, in contract language.
Let me be clear: I am not saying these sites are out to screw you. But because Zuda and Duck are owned by big corporations, they have attorneys whose job it is to write contract language that is as favorable to the site owner as possible. In my opinion they go too far.
I regard Web Comics Nation, Comic Space and Online Comics as safe, and in fact, I post my own comic, Scratchin Post, on all three. I have not spotted danger on any of the other well-known sites, but I have not reviewed them as closely.
Of course, we have one weapon that most people exploited by contract law don't have: we can draw savage and unflattering comics detailing the conduct of the offending fat cats. The pen is mightier than the briefcase.