Firstly, the audience will usually not notice when you or a bandmate messes up, so dont look disgusted or angry at the person at fault as this will give it away.
um, what I hate is a band that plays for too long. New bands that play sets for over half an hour annoy me. No one is able to remain interested in music they dont know for too long...If you are not yet established, and the crowd dont know your songs, keep your sets down to four of five songs, leave the crowd wanting more, rather than bored with a band that they enjoyed for the first few songs.
another pet hate of mine is when the guitarist (for some reason only we do this) plays a lick/line/progression from the next song whilst the vocalist is talkting between songs...unless you have a planned intro, or interlude, dont play the riffs from your songs!
oh, and start your gigs on time!!!
[FONT=Century Gothic]Hope is when we feel the pain that makes us try again[/FONT]