Being someone who's Modded OC and prefers to play OC over Cannon I strongly second sharizard.
As a player: I look for how well the game is thought out (technicality/mecahnics wise), how the rules are laid out, what type of characters are allowed, what types of characters are in the game all ready (don't want to step on toes), how much flexibility is allowed in the character creation process, how much the mod(s) is/are willing to work with their player, how friendly the mod(s) are, how much they know about the world they've set up, the activity level of the comm and how friendly people seem in regards to being contacted/when they are contacted. -- Wanted characters/pre-made concepts can be nice and I've met some players will basically only play 'wanted character types' but personally I'll take a glance at the 'wanteds' but normally find them to restricting. The very general 'wanted' ads are the ones that are most likely to gain my attention. (Once again, like the above example given by Sharizard)