This has me going back over the past six years and realizing I've had to relearn tanking four times now. I had to learn how to do it originally in order to start working on Molten Core for my then-guild, and then I had to relearn it in The Burning Crusade (and actually, I had to relearn it twice there, thanks to the awful implementation of rage normalization for warriors and our astonishingly bad AoE threat that whole expansion). In Wrath, I didn't so much relearn it as suddenly find it much more efficiently designed and fun. Finally, Cataclysm has me tweaking how I tank, but I can't really argue I've relearned it from the Wrath era so much as simply refined it. Meanwhile, I've also had to relearn the DPS side of my class every expansion, in much the same way.
All of this learning has been done on the fly. To paraphrase a famous quote, World of Warcraft is vast and deep, and I'm swimming forever. Most certainly, there are sources to go to for players who want to learn a new role (one of them being this site), but there's only so much you can be taught before you have to hold your nose and jump in. This makes me wonder two things. First, is there more that the game could do to teach those roles, and two, would it be beneficial or harmful to immersive gameplay if it did?