My problem with LotR (the book, I still haven't seen any of the film) is that I know it too well. That's unusual for me, because I can normally reread books even when I've read them 50 times, and I've only read LotR about 5 times, but the periods between me rereading it get bigger exponentially. I think it's because it /is/ so well written (in my definition of the term -- it induces not just secondary belief but primary belief in me) that I don't need to reread it.

(A funny: the first time I read the Silmarillion, the first part about the creation of Ea, my thought was "Ah, so that's where the Norse myths came from!" See what I mean about primary belief? JRRT wrote them better than the skalds did...)