After the success of the first two Spider-Man films, Spider-Man 3 represents Sam Raimi's inability to overcome the biggest hurdle of a third movie, a hurdle that Alien 3, The Godfather Part 3, RoboCop 3, and the list goes on could not: studio interference.
Although I wish that would be the only thing wrong with Spider-Man 3. But almost every storytelling decision - from the creation of Sandman to Peter instantly falling in love with his new costume to that thing that was never actually called Venom - is just plain stupid. Right from the get go you hate Peter Parker.
Studio interference did not mandate the jazz bar dance scene. It did not have the Green Goblin 2 storyline make absolutely no sense whatsoever. It did not create that stupid love triangle between Gwen and Mary Jane, betraying everything Peter Parker is supposed to be about.
These were fundamental issues in the script and direction the film took. These were issues that doomed the film for failure. That's why this movie sucks, not just because they butchered Venom.