It's pretty obvious early on just why this book has been challenged or banned so many times - it's, quite frankly, the sex, and this novel is quite frank about sex, specifically as it concerns teenagers. Forever... is a novel about a first-love relationship, and writing about that without factoring in sex would be, frankly, dishonest. Even in these days of "abstinence-only" sex education, sex is going to be a factor in teens' relationships; at the very least, there will be conversations about why they're not going to have it or how far they'll go with each other without actually having it.
I first read Forever... just a few years after its original publication (plus a few more times after that), and I think it was very influential for women of my generation. At the time, it seemed like one of the most "adult" young-adult books I'd ever read, and not just because of its subject matter (frank, yes, but all things considered, not overly graphic); it didn't shy away from the complexities, it didn't talk down to its readers, and it's one of the first books I remember reading that didn't have a neatly tied-up ending (which has become something I appreciate in novels, most of the time).
Given my prior relationship with the book, I'm finding it difficult to be objective about how it's held up over time; enough of it has stayed with me that reading it again was a bit of a time-travel experience, and may have made it seem more current and relevant than it really is. And to be honest, there are some aspects that seem a bit dated. But the central themes of the book still seem to matter: sex and emotion are tied together, and early sexual experiences ideally happen within a loving relationship; the people in the relationship are well-matched and don't play power games with each other; communication is important; becoming sexually involved is an adult-level decision, and an adult level of responsibility is attached; assuming that the course of your life is set forever by the time you're eighteen is a risky proposition.
Having said that, there were things in the writing itself that got to me on this reading that I don't think I even noticed as a younger reader. There are aspects of the novel I still appreciate - Katherine's relationship with her parents, the mostly non-judgmental attitudes, the matter-of-fact approach to sex and contraception ("safe sex" meant different things in the pre-HIV days when this book was originally written). However, while Blume's tracing of the trajectory of Katherine and Michael's relationship is convincing, I didn't really feel that the characters themselves were all that developed, and the excessive use of ellipses in the dialogue just irritated me. (Granted, I'm an offender in that manner sometimes myself, and I know people tend to pause when they're talking; but I don't know if it's necessary to convey that in writing their talking.) And seriously, has any guy ever given his penis a first name of its own? "Mr. Happy" seems to be good enough for most of them...
I think that Forever... is still an important book, and I'm glad I revisited it during Banned Books Week. I think it's a modern classic that deserves continued reading, and I think that it will continue to be banned and challenged for as long as it's around.