Well, at the risk of injecting some serious discussion into a rather silly thread...
I can understand disliking Rocky Horror, but not lumping it in with Grease, Dirty Dancing and so on. That's like trying to file Airplane with Airport 74, or Priscilla Queen of the Desert with Mad Max.
The plot of Rocky Horror is paper-thin, and the effects are cheesy... but that's all part of the parodying of the low budget horror genre, the genius of the film is in the acting, the way it deals with sexual (and indeed transexual) awakening, throws every horror cliche into the mix, has some great tunes, hilarious gags... and the fact that it's the only socially acceptable excuse for men to wear women's underwear in public might have something to do with it too.
Performed live, there is always room for new twists, and of course audience participation. I loved Nigel Planer as the Narrator switching into character as 'neil' from the young ones when heckled:
"oh no! I'm being hassled by a chick!... It least I think it's a chick..."