As a guy in his mid-20s I've been ambivalent about Wicked since my first time hearing the recording. I just expected to like it a lot more than I did.
My younger sister, while 23, is right on board with the teenage girls that love this show, and for a lot of the same reasons.
What I find interesting is that the themes that supposedly draw these girls to Wicked are almost the same as those in Hairspray. Odd girl out shines through by being herself, ultimately beating out the pretty, popular girl and winning the heart of the handsome guy. What I think separates the two in the minds of many young fans (or at least my sister) is that Hairspray is so explicitly about adolescence, and that seems far to simple and normal for your typical 16 year old drama queen. It resonates on a much deeper level to identify with Elphaba than Tracey. So Tracey's fat, big deal. Elphaba is GREEN, and brilliant, and totally misunderstood. It's more fun to identify with her than the girl who is pretty much just like you, but in the 60s.
I also work as a psychologist with adolescents, and sometimes I find it interesting to ask what kinds of music or media really touch them. Usually it's something hip-hop, but regardless of the source what they usually discuss is some abstract, over-the-top lyric that somehow relates to them personally. In the case of Defying Gravity for your average 17 year old theater enthusiast it might be something like this...
"Too long I've been afraid of Losing love I guess I've lost Well, if that's love It comes at much too high a cost!" (real-life translation: I can't get the popular boy to like me. Oh well, boys suck anyway)
"And if I'm flying solo, at least I'm flying free" (real-life translation: I don't have a boyfriend, but I'm so brilliant and talented and empowered that I don't care)
"So if you care to find me, look to the western sky!" (real-life translation: if anybody needs me I'll be up in my room sulking and listening the cast album of BKLYN)