Other bands being slow to set up and tear down...especially tear down. You finish your act, you move your ass as quickly as you can off the stage...THEN you start milling about and bullpoopieting with people.
Bands bringing a complicated rig to a poopiehole venue for a free gig.
Bands bringing amplifiers that outdo the P.A. OK...so you actually WANT your band to sound like crap?
Noodling, excessive testing, taking forever to set up your pedals, etc. Get that stuff squared away OFF the stage, so all you do is pipe a cable in and a cable out of your board.
Playing along with the house music...especially when you don't know how to pay the damned song.
Drummers who won't share their kit...yet also take their sweet-ass time setting up and tearing down.
Bands who pack up *completely* while still on the stage. NO! You shuttle everything off the stage as quickly as possible, then you wrap up the details off to the side. You don't stand on stage wrapping up your cables, pedals, putting your amps into cases, removing and packing your cymbals, etc. No cases on the stage, and no assembly or disassembly of your rig/kit on the stage.
Set lists left on the stage.
Drinks left on the stage.
Spills left on the stage.
Load-in times that are idiotically early.
Bands that play encores without sufficient demand for them.
Bands that push the start time because "nobody's here yet." Too bad! Go on on time!