Imitation of Life (and the Reveal album itself) is pretty much the opposite of rock.
But their roots are very much in rock. The 80s were mostly rock. Monster is rock. New Adventures in Hi-Fi is rock. Accelerate is rock. It's a pretty loose genre.
I guess that's where 'alternative rock' comes in. A genre pretty much invented by R.E.M., and without them, there would be no Nirvana, Radiohead, Green Day (pre sell-out), Muse, etc. OK, none of them are my kind of thing (I don't mind a bit of Radiohead, but they're not exactly favourites of mine), but that just shows how far R.E.M. reached - in a genre I'm not really into, R.E.M. were diverse enough to reach me and penetrate into my depths - no matter what kind of sound, every album I see as a classic (barring Around the Sun).