...is that her music is crashingly dull. "Wrecking Ball" isn't a bad song, but it's just so bloody ordinary.
What I find offensive about her weed-smoking is that it perpetuates the myth that drugs give you creativity, or expand your horizons in a meaningful way. The Beatles, Charlie Parker, etc made great music because they were talented, not because of the drugs they did. (I knew someone who would always say "man, I wonder what drugs Dr. Seuss was on when he drew that shit." Sigh)
Frankly, I still think that in 20 years Miley will be headlining Branson, voting Republican and attributing her current actions to the excesses of youth. But maybe I'm just cynical because I've never smoked the good shit.
(ETA: imagine if instead of weed Miley sat down with a shitload of books by Chomsky, bell hooks and mainlined Democracy Now! That would make for interesting music...)