cleanliness, and customer service. if the place smells funky or looks dirty I walk out dont care what they have. even if the place is clean but the tanks look iffy out I go. And if (depending on the customer to rep ratio) I am not greeted or asked If I can be helped within 3-4 minutes I leave (if the place is dead it should be almost immediate, if its packed and the employees are swamped it could never happen and I would understand,)
they could have a lousy selection but with nicely maintained tanks and good customer service I will go back everytime hoping for something cool. you could have the coolest coral in the world at a great price but it'll sit in that rotten tank until the end of time before I buy it, if the tank looks poorly maintained.