I'm impressed by someone who can master a variety of techniques.
Good playing overall with something special that makes them stand out from the rest.

Fast shredding and stuff doesn't do anything for me at all. Usually cause the second they start playing chords or a lead with any type of feel in it, it shows that they don't know what they're really doing.

I guess best of all I like clean playing. Somebody who knows how to properly prhrase chords and leads so that they blend properly. (ie, ...I can't stand it when players use open strings that aren't even in the right key when they're trying to play smoothly between chords, or guys that can't do vibrato and bends properly)
I also like it when a player knows how to palm mute properly and can play without scratching their fingers all over the strings.