Adult Alternative isn't quite the same as Classic Rock. I mean it is like Classic Rock in that it is a genre completely invented by the radio stations that play it.
It isn't Classic alternative, but alternative and pseudo alternative music as well as mainstream music that is a bit too blase to be top 40, suited to the tastes of adults in their late 30s and early 40s.
An adult alternative playlist looks something like this:
Bjoerk
Radiohead
Wilco
Morphine
maybe some blues- nothing too good though, maybe some B.B. King
stuff off of Robert Plant's solo albums and other new agey bullshit
maybe some Joy Division thrown in there
some Morrisey
some Belle and Sebastian
KT Tunstall
check out this website:
http://www.wyep.org look at their playlists, album reviews. You'll get the idea.
You will never hear anything angry or energetic sounding on an Adult Alternative radio station. So no industrial, punk, hardcore, post-hardcore, noise rock, etc. Maybe some art rock, as long as it is kind of mellow.