I think one of the main reason these guys sound so awesome is because of the chemistry shared between the 3.
Duff - Bass, and he also knows how to play drums, so can adjust his playing according to what matt is playing, Also, Plays guitar so manages to create a big groove for slash
Matt - Awesome Drumer. plus he can play the guitar (he made the riff for SMF) hence he also knows hot to create a groove for slash...and has some awesome fills, rolls, n melodies
Slash - Amazing musical sense. Can adapt to lots of styles, And since he basically is a rock guitarist, he gets the perfect platform.
While ppl say that dave is good i think he isn't that good.
If you hear AFD, You can hear the rythm guitar clearly(but not overemphasized)..it basically really sets the pace n gets u into the song....
whereas Dave just makes weird noises n there's no actual sense of rythm...i miss Izzy