Most important (after getting paid) is low minimum deposits. I only like to deposit what I'm going to bet and, since I usually only bet $25 a game, that knocked out all the good books that make you post up at least $50.
Of course, I still make a bet with either Greek or 5Dimes, but I have to really love their number to post up twice my normal bet size.
Of couse, they have to take free neteller deposits. They also shouldn't have any strings attached for withdrawls, but can see past this in the right circumstances (once again, 5Dimes and their 15 dollar fee Tuesday-Sunday).
Other factors I look at (but aren't dealbreakers) are:
* - Easy to navigate site: Some of these sites go hogwild with their use of frames (Hello, BetGameDay). You're bookies, not Microsoft. Keep it simple.
* - Automatic Credit: If I've decided to play a game at the last minute, I can't wait for someone to manually credit a neteller deposit. SIA and BetGameDay automatically credit my account when I deposit with them. I tried getting down on a hockey game last week with 5dimes and posted up with 20 minutes to go before the game. After waiting 15 minutes for the money to be manually credited, I cancelled the transfer and moved it to SIA (who got my money when my side lost).
* - Grading: As much as I like to play at SIA, I had to drop them entirely during football last year. They couldn't grade games that would end at 8pm and that kept me out of west coast games. This has never been an issue with anyone but them.
I know this list may sound nitpicky to some people, but everyone has their pet-peeves, and these are mine.