Pommy, you change your sig almost everyday... :O
Well... a good concert to me is one where the artist puts all his/her/their emotion in the songs they perform. doesn't matter if they play their instruments, sing and dance, as long they do their thing with all their hearts. You can see when someone feels forced to do something live just to get the check. Previous training also helps a lot and the interaction with the public is also very important. That's what makes a good concert in my opinion.
Oh, and props to Madonna for doing so much... singing, dancing and playing the guitar...