Clean tanks.. no dead fish floating around.. i understand that sometimes fish dont make it, but a good LFS is keeping an eye on their stock and if a fish dies it gets removed ASAP, Im generally looking for a store that sells both FW and SW stock, if i'm checking out a new store or one i haven't been to i always ask a few questions i already know the answer to in order to gauge how knowledgeable the staff are... nothing annoys me more than when people say.. well the guy at the fish store said (insert wrong information here)
really though clean healthy looking tanks and knowledgeable staff are what makes a store for me.