I choose sites 5 or 6 years ago based on the ergonomic feel of the tables. This was before the UEGIA, so the whole internet poker world was still open to me.
At that time I preferred Paradise Poker. And Party Poker
. There were many sites that just didn't cut it for me from a 'comfortable' POV.
The last holdout for those uncomfortable sites to sit at seems to be Bodog. I only play there when I have to, and thus seldom do good there because I don't like the ergonomic feel of the place.
So my criteria was comfort. As I built a bankroll, other factors, most importantly security, came into play. Then opportunity (ppl per site, and variety per site).
But I still play the sites I feel most comfortable at.
FWIW, I have got most of my sites looking almost exactly the same using each sites table view options. I prefer a Stars shaped table, with the darkest background I can get. Cartoonish avatars for me are fun, but absolutely unimportant. Balloon chat is a waste of computer resources.