I love Buffy, but she might be a little bit young depending on a) how creeped out she gets with scary stuff and b) how comfortable you are with the fact that it does directly address sexual themes from season 2 onwards (losing of virginities, coming out as gay, and even in season 6 some rape/sexual deviancy stuff).
How about Gilmore Girls? It's a great series from the Buffy era, about a mum and daughter who are both really spunky and intelligent, and have a really close friendship. Great sense of humour and mix of drama/comedy. My only fear would be that it seems quite girly on the surface (eg on the DVD cover) but really its not girly underneath, it's primarily about celebrating women as strong, intelligent and not needing men. And I think it's definitely age appropriate for an 11 year old.
Other series that spring to mind with more of a sci-fi/fantasy element but lots of plot-driven drama are Smallville and Roswell High, but its been so long since I've seen either that I can't remember if there's much sex/scary stuff in them. They're both about high school kids, so I doubt it would be that bad.
Veronica Mars is another very clever and enjoyable high school series, with a fantastically strong un-girly female lead. Again it does address some sexual stuff, like the fact that one of the running plots of the first season involves the 16 year old lead thinking she may have been roofied and raped, but it's not visually explicit and its dealt with with quite a light and intelligent hand IYSWIM.
Other series bouncing around in my head: Roseanne, Big Bang Theory and Futurama
Wow, I really want to snuggle up under a blanket and spend a few days watching back to back boxsets now!