Okay, this is probably going to sound pretty ignorant but it's something I've thought of for a while now.
I think a small part of Superman Returns' so-called "failure" (which I don't believe in) and Pirates of the Caribbean: Dead Man's Chest's success can be attributed to the fanbase. Honestly, the night Superman came out everyone was *****ing about "they did this wrong, this isn't right" on IMDB, but when Pirates 2 came out there were LEGIONS of fangirls on there flooding the boards with "OMG THIS MOVIE IS AMAZING!"-type posts, and being completely rude to anyone who pointed out the flaws of Pirates.
And because so many people use IMDB and various movie websites as a resource, I think it had a very small but still impacting effort on the box office. If you can't decide between two films, and you see a bunch of people *****ing about how bad one film is (and really, most often not even willing to point out even one redeeming quality a film had) and on another you see a ton of people raving about it, most people are going to be inclined to go with the one getting rave reviews from their peers.
That being said, at least Superman Returns got a generally favorable critical reception and Pirates 2 not so much. I also have heard better word-of-mouth about Superman from NON-FANS who are willing to take it for what it is (and not complain about the minute details) than I have Pirates. Yet the Superman internet community as a whole has this feeling of "Superman Returns sucked, everything was wrong, just reboot, etc., etc., etc.", because many people on this and similar boards are - much as I hate to say it - fanboys who will always find something to complain about.
Bottom line: Superman Returns at least returned a profit with all things considered.
Wow, that was pretty incoherent, sorry. Just kinda wanted to state my opinion.