for me a gig is generally good as a result of 3 things.
1) small venue, i find huge stadium venues are too impersonal, and i like the smaller venues where there is more interaction between band and crowd. sure there wont be much interaction with a BM show in general but other metal acts like say MF or bands in other genres have front ment who handle the crowd really well (example: dee snider)
2) the band enjoying themselves/geting really into it, if the band doesnt get into it i find that the music can often sound quite boring and forced... This reflects onto the crowd and the crowd often doesnt get into it much then
3) atmosphere/how much the crowd gets into it. i find if the crowd gets really into it then the atmosphere in the room seems to be that much better. usually the band reacts upon this and has a better time relaxes and the music starts to sound better (generally anyway)
saying that I am quite picky with which shows I go to...