The following is a spoiler, of a sort; kind of a meta-spoiler. But I nonetheless claim that almost everyone will enjoy Superman Returns more if they know this than if they don't know it; knowing this will help you set your expectations appropriately (and most of you probably know it already anyway). However, I know some people won't want to know it, in which case don't read the rest of this entry.
The key thing I think you should know ahead of time is that this movie is much more closely related to the 1978 Superman movie than to any of the comic-book versions of Superman.
I read somewhere that Bryan Singer is not a comics fan (which makes me even more amazed at the care he took with the first two X-Men movies), but is a big fan of the 1978 Superman movie. And I'm very glad I knew that going into this movie, because I think I would've been really disappointed if I hadn't known it.
I didn't see most of the sequels to the 1978 version, so I don't know how closely related this one is to any of them. But there are quite a few elements and approaches here that come directly from the 1978 movie, that I've never encountered in any Superman comic.