I don't know what went wrong with this film in the wider world. In the grand scheme I don't think it's at all that bad - I know I've seen considerably worse films and worse superhero films before and after it.
The sheer quality of everything, from the spot on titles, the countless nods to the first two films, the gob-smacking effects, down to the use of Marlon Brando, a neat recap to explain why films 3 and 4 (are wisely) removed from the story and great casting (except Bosworth - a good enough actress but definitely just way too young looking for Lois Lane it has to be said) makes this film reek of class. Routh may not have done a lot since but he does an admirable enough job of taking over from the late Christopher Reeve. The story itself is simple but powerful as both Clark Kent and Superman try to adjust to a world (and a Lois) that got used to not having them around and moved on, while the villainous Lex Luther continues his fascination with land acquisition!
There are downsides (the flashback to the young Clark could probably have been cut, the finale is overlong for instance) but I think it's a great couple of hours entertainment, and given it was not at all badly recieved back in 2006 when it first came out I'm not sure where some of the dislike comes from. Director Bryan Singer was a self-confessed lover of the Donner films and maybe his love clouded some of his judgement in some areas but at any rate I still enjoy it immensely.