i obviously go to quite a few gigs, but they tend to be 'big' name bands rather than small pub/club gigs of the type I usually play.
with the big shows, you are quite far away from the guitarists anyway, so its hard to be too judgmental, also its ridiculously loud. i tend to take in the whole theatre of the thing.
on the occasions i do see a smaller band, i do generally check out their gear and such like pretty closely. its funny, but i've never judged another guitarist in that situation on technical merit. i generally try to judge their material in a more artistic way. of course, cover bands would be different in that case to musicians playing originals.
i do hear bum notes very easily from other musicians, but since i play a load of them myself, i'm not hard on them about it ;)
and i would certainly agree that i take comments from people with a big pinch of salt. its nice to be called a good player, but i know where i'm aiming for, and i'm still along way off!! i can widdly-woo with the best of them though ;)
guitarists do indeed advise you to make the guitar sound louder, generally i prefer to sit nicely in the mix and not be too loud, except during a solo. i ask other musicians for advice on that, but not guitarists.. they always seem to want it louder.
edit: finally it is good to talk to other guitarists after shows to compare notes and ideas. i have offered what i felt was constructive advice in the past, but i'm no jimi hendrix myself and know what its like to be starting out so i would never do anything to dent someone elses confidence. generally people go out to see a great show, and i don't your average punter cares about a few wrong notes, and even if they are heard, they are quickly forgotten.
i hate receiving too much lavish praise myself after a show, i think the best compliment you can get or give to another musician is 'good job!'