i'm gonna add to what others have said, and say that a good guitar tone - along with fitting the genre and rest of the band - is ideally unique in character, and easily identifiable by the listener. i think this is what's lacking in so much of the heavier stuff that's coming out now...few bands have that signature sound, where you can hear literally a fraction of a second of ANY song, and instantly know who it is.
i don't know about anyone else, but there's dozens of artists who i can pick out by hearing even a single guitar note, and know for a fact who it is, because their tone sounds like no one else. even if it sounds sorta shitty, like pantera or old sepultura, it is what it is, and you KNOW what it is, which is what excites your brain when you hear it. now get 100s of dudes reamping their shit thru the typical TS/5150/mesa setup, and play all their albums/songs in random order...none of it will stick with you, unless the music itself is exceptional. it all becomes a big monotonous blob, and your brain just stops paying attention, whether you mean it to or not.