What makes a guitar tone good?

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.

Perhaps it's not just the tone, but the bands that aren't so unique either.
 
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.

This!

And also agree with MotherEel.
 
can't we just keep to "beauty lies in the eyes of the beholder" ?

That's how we end up with snare sounds like St.Anger. :) But still... that was at least original.

Perhaps it's not just the tone, but the bands that aren't so unique either.

You're right! IMO there are too many bands that want to be another band. F.ex. now that Periphery is getting more famous there are hundreds of djent bands popping up and they sound exactly the same. Axe-Fx FAS Modern with screaming and clean vocals in choruses and Superior Drummer instead of live drums. If Periphery was just like Meshuggah they wouldn't have made it this big. Why don't people come up with the next step? The world doesn't need another Periphery. The world doesn't need another Justin Bieber. One is enough. :D