Yeah, relationships would be the main one I'd say, but social class is really good to talk about too, as well as womens roles.
Social class - Even from the opening scene of Clueless you can see that the kids are wealthy, popular, having fun, etc etc. And Cher's "I actually have a way normal life for a teenage girl", juxtaposed with the shot of her looking through her computer controlled wardrobe.
You can also talk about the scene with Mr Elton and Emma when he tells her that they should be together and compare it with in Clueless, Elton and Cher. Those scenes are really similar and really well done.
Womens role - My thesis in the last essay I had to write on this was that although women now have more to do (eg: shop), what they do isn't really much more important than in Emma's time. Whether or not it's true, that's what I saw in Clueless .. Women still think of themselves largely in comparison to men, and are still vying for relationships with the "right" one.
Whiterabbit said: "The marriage at the end of 'Clueless' shows the lack of importance.." ..You could also say that marriage has been turned into nothing more than a fashion show. Instead of talking about relationships and love etc at the wedding, Cher and her friends are organising what they want to wear. Shows what we value today, etc.