Spider-Man 3 terrible for two reasons (at least): Dancing Emo/KD Lang Spidey, and the Exposition News Channel setting up the third act. There's many, many more reasons but those or the two bigger ones, for me.
Those strike me as fairly minor issues compared to the laughably atrocious Harry Osborn amnesia plotline, the fact Green Goblin II is in fact aimed at an extreme sports demographic and now called "NuGoblin", Toby and Kirsten's beyond awful performances, the incomprehensible unnecessary new backstory to Uncle Ben's murder, the "dark" symbiote from the comics now turning Peter in to Austin Powers, Harry's butler's random revelation he'd been holding back for no reason and of course, Venom being built up to for the whole movie only to be killed after 5 minutes. Where he was almost always unmasked because that's how people roll in the Raimi films.
None of the aforementioned storylines fit together in any way. It's a film comprised of utterly idiotic elements clashing together and somehow making eachother more idiotic. Calling Spider-Man 3 "better than amazing" has caused Sergei Eisenstein, Ingmar Bergman, Orson Welles, John Ford and Jean Renoir to spin in their graves so fast, they're now tunneling through the Earth's crust.