I think there's a lot of variation in what kids are into, both regionally and from kid to kid. I've subbed in middle school and my kids there were interested in very different things than my own 12 year old, for the most part. My daughter is into ballet, braodway musicals, alternative music, blogging (mostly livejournal), hanging out with friends, going for coffee, fashion, knitting, boys, AIM, reading, hair and make-up...
The kids where I subbed were into raising animals for 4-H, football games, music, basketball, hair and makeup (just the girls on the makeup), fashion, and cool vehicles (mostly the boys, and mostly kinds of trucks and truck accesories).
I think the common denominator is music... if I let them turn the radio on in the background, a lot more work got done.
My junior high kids were all fascinated by the whole musical theatre thing, too... I could get a lot of leverage out of singing a song from Bye, Bye, Birdie.
Mostly, I think middle school is the age of transition, where they're working towards being "grown up" like the high schoolers but are still mostly unsure of themselves... they go back and forth from "cool" to "kid" all the time. Not losing face is very important at this age, probably more than any other. They still can get really excited about things, and sometimes they're really funny, but laughing is a big, big mistake because they don't have much of a sense of humor about themselves at this age.
There's also a huge variation in maturity at this level... in one 7-8 grade class, I had one boy who was under 5 feet tall and who had a little plastic toy and kept "shooting" the other kids, making his own sound effects as he did ("Bam! Pshoo, Pshhoo - POW") and another who wouldn't have been out of place in a college classroom. It's tricky to treat them all repectfull but also acknowledge the different levels that they're at.
I really love middle school best, though. It's a fun age...
Single mom to Rain (1/93) , grad student, and world traveler