A good show for me is when I don't get bored after 15 minutes. Not a lot of bands seem to have that quality, but a band that really surprised me on that level recently is Dream Theater.
1st time was at a festival in Belgium, in August or July, was pretty good.
2nd time was about a month ago in Los Angeles, they just blew me away. It was a very simple, no-bull**** kind of show. A terribly good sound, great energy, and as opposed to so many bands out there these days, they don't rape their own songs live. Which makes me think of something, it seems like most bands these days are "cd-bands", they don't have the skill or motivation to actually pull it off live. That's why I appreciate band who DO pull it off live so much. We need more of those.