I totally agree.
I've said it before, Spidey 3 just proves that there can be too much of a good thing. If you objectively look at all Spidey films, they all have their share of flaws, most of them much more serious than Venom's use of "I" instead of "we" or half a minute of tension-releasing dancing. Spidey 2 has some pretty big plotholes, yet nobody cares. At the time we were too excited to notice them. With Spidey 3, fans expected exactly what they had in their heads and when they got something a little different, they picked up their forks and torches.
Heck, with whatever flaws it had, Spidey 3 was an involving, emotional film, while not lacking at the action department. People are praising Iron Man, which, while faithful and entertaining, lacked ANY kind of emotion. It was a flat action-humor-action-humor line. No investing in the characters, no drama, no suspense. Sandman had little character development? What about Stane? Where was his development besides being a slimey SOB?
Anyway, I digress. My point is, whatever flaws Spidey 3 had, most other films (not just comic book films) also have in spades. It just had the bad luck of being the third in the series and people got kind of full. I seriously doubt if there IS a third film curse, or people just grow too satisfied and picky after the second film. I know some films, like Batman Forever and Robocop 3, are beyond bad, but even Godfather III was badly received.