Being a member of many different design communities, I have to say that a few things I like to see before I enter a contest are the following::
Guaranteed winner and payment
Contests that don't run to long (ending in 7 days is ideal)- Personally, I don't enter contests that run longer than 7-10 days.
No member manipulation functions (Such as allowing members to vote for other members) - 9 times out of 10 this promotes manipulation of the voting system in an attempt to get their designs higher up on the charts and drown out others (even if it's the worst out of them all)
Moderation / Administration participation in policing - This is VERY important to me, as it's one thing to lose a contest fairly And another thing to lose a contest to someone that ripped off a copyrighted / clipart / stock design & tried to sell it as their own. It's imperative that a site has a REPORT feature on each submission & that the staff conducts research into the infringement report. Some reports will be bogus, made by those that want the best design out of the way so they have a better chance to win, however, with the staffs research a fair decision can be obtained by looking at the evidence.
The above are just the first things that pop into my head when I'm looking at a new contest site. The more fair & policed a contest site is, the more designers & clients you will attract.
Just my thoughts, hope it helps