I've only ever looked/started original bands, so with that caveat...
+ Inability to improvise. I find that if the band just jam and play off each other, then they're not that great.
+ Getting drunk/stoned/high during the audition - concentration and hearing goes out the door when you're intoxicated.
+ Not offering to help with gear - that means they're a bunch of douchebags.
+ If you can't have a conversation with them between songs because they're really quiet or have nothing in common with you, then it's probably not going to improve with time.
+ Inability to improvise. I find that if the band just jam and play off each other, then they're not that great.
+ Getting drunk/stoned/high during the audition - concentration and hearing goes out the door when you're intoxicated.
+ Not offering to help with gear - that means they're a bunch of douchebags.
+ If you can't have a conversation with them between songs because they're really quiet or have nothing in common with you, then it's probably not going to improve with time.