I agree with most of what others have said here.20x20 is standard in our area as well. We have a 12' pop up camper that extends to about 20', so I like to have a 20x30 space if at all possible so I can keep my truck nearby.
Most of the comps we've done (20 in all so far...god are we newbies!) there are usually enough outhouses, but sometimes they aren't dumped frequently enougn.
There is usually, but not always, accessible water. The better ones stretch what they call hose trees all around so you can usually be within 50' of one. They often suggest you bring along a splitter to help share. Mine was stolen at the last comp!
Power is usually 20Amp, if they have it, occaisionally 30. We bring a generator as it's much more reliable.
Ice, for sale at a reasonable price, in my opinion, is nescessary, especially where once set up you aren't allowed to leave
Real bathrooms, sinks, and showers are a luxury that we've seen available at some, but not many comps, but they sure are appriciated where they're available. We keep a coleman instant water heater which has doubled as a shower on occaision, you can pick up a pop-up shower/bath enclosure at Bass pro or other sporting good stores for not too much dough.
Most comps don't seem too concerned with grey water around here, actual RV dumps are another story.
I always welcome easily accessible ash barrels. If they are not closeby, I'll usually dump my ashes into a disposable full foil pan full of water, as I don't want to be carrying a bowl full of hot coals further than 10 or twenty yards. Grease disposal is far too rare at the comps around here, at least in my opinion.
coffee and donuts, or some sort of breakfast is also a huge plus, but not always available.
Finally, as to the prize pools, I just think that a fair share of our entry fees be used for prizes, and not all as profit for the organizers. Usually, we decide before going if the prize money offered is worth the trip to us.