1) Decent support
2) Prompt and reliable cashouts
3) 4 colour deck - important for those of us with less than 20/20 vision
4) Bet and pot amounts shown, with no need to hover a mouse
5) Usable slider or ability to enter bet amounts
6) Ability to turn off chat and/or avatars
I hear good rumours of Canbet and Poker.com, by the way.