Limits is a good one. I've never been topped out but have heard a few guys on here say their accounts have been suspended or limited and never really understood why?
The speed of the site itself also might be worth looking at. Some sites grind to a halt on Saturday afternoon when I go to put my football bets on.
BetFred seems to struggle particularly.
Also, for folks like me, are they Mac-friendly? Some ain't.
What about In-running markets? That seems to be more and more popular.