Rereading books you read 20 years ago

In the past couple of months I have started rereading books I read last in the 1990s and liked a lot then. The surprise and excitement of discovering a new world is less, of course, for I am already familiar with the worlds in those books. What surprised me the most, is that some books still hold up while others have become boring, bland, or otherwise uninteresting.

For example, I was unable to even get into Williams' Otherland series. And I devoured Feist's Magician almost like I did when I was in my teens.

How do you experience rereads from your youth? What writing characteristics makes a book eternally fresh or almost immediately dated?