Seven reasons Green Day's Dookie is destined to be an overlooked pop gem:

1. Multipart harmonies make singing along fun, even for grumpy teenaged slackers, or people who were in 1994 and now don't want to admit it.
2. It's ambigious, but slyly so: unless you're one of those nerds who learns all the words to songs, or a girl, only about a third of the phrases come out, when you sing them, as the Queen's, or President's, English. That's OK - when you sing to yourself or with friends, the mumbling makes you sound more like Billie Joe.
3. Punk + Pop = Short, Memorable Songs
4. Loud Guitars and Drums and Songs About Masturbating and Being Bored and stuff make the music less palatable to Pop fans.
5. High Sales and Worldwide Fame make the music less palatable to superhep Alternakids of Yore who have discovered much more Punk stuff with more Street Cred.
6. "The sophomore slump:" even though it wasn't Green Day's second album, Insomniac and later discs sold less and thus tarnished their popstar reputations, which casts them as one trick ponies who maybe don't deserve canonization.
7. "Hits of the 90s" compilations (already appearing in stores, oh dear God) that want to display a rougher side will sandwich "Longview" (or more likely "Welcome to Paradise" since it's not about jerking off) between "Smells Like Teen Spirit" and some crappy Matchbox 20 song. Unfortunately, these will be FAR outsold by compilations made for sentimentalists and office-workers, including That Song With Acoustic Guitars from Nimrod, whatever it was exactly, but it was so pretty, wasn't it?