For me it's a combination of things, a lot of these have been mentioned already in this thread. These here are my big three.
Overall musicianship - do they sound good live, doesn't have to be a carbon copy of the album, which I don't really like anyway, but does it at least sound like the band/song I'm familiar with. Are they playing in tune/right key? Depending on the band are they taking a few chances and jamming a bit? Playing the songs in a different arrangement. the right bands can do that well abd give you a new twist on an old classic which can be very cool.
Setlist - There better be a few deep cuts for the longtime fans in there, I don't want a greatest hits type show unless it's a farewell tour type of deal. Nothing bums me out more than a predictable set list or one that doesn't change from year to year. Ozzy and Poison are notorious for this, neither have changed their setlist for a show in over a decade.
Showmanship - Doesn't necesarrily have to be tons of pyro and lights or theatrical in nature but it does have to be a show. The band is there performing so they better be performing and entertaining my ass otherwise I may as well have stayed home and listened to the album. A concert needs to be visual as well as sonic. I've alwayd agreed with a quote of David Lee Roth's on that "Go big or go home. You need to make sure you're entertaining the cat that's in the last seat in the last row. He paid the same hard earned money that the cat in the front row did. So you'd best be playing up to him and make sure he's got something visually stimulating to look at as well as being sonically entertained. Now that doesn't mean that you rely on the theatrics cause let's face it. If you can't go out on stage in a pair of jeans and a plain white T-shirt with one spotlight and figure out a way to entertain then you don't belong there." Very true.