Audible vocals are the biggie for me. If I have to strain to hear what the singer is saying I lose interest. More so on recordings rather than live shows. At live shows it is expected and common. Besides some venues just have crappy PA's. I know quite a few here in Austin that do. Not always the bands fault. To me the vocals are the most important part of a song. This is from a music listener standpoint, and not a musicians because I can't play music. I have noticed by working with musicians over the years that the two are very different in opinion.
Secondly I pay attention to the guitarist. If he / she is down with some groovin guitar work, rippin solos, fuzz buzz, I am a fan.
Just a personal preference.