It is by no means the worst movie ever created, but when compared to Spiderman 2 it just wasn't nearly as good.
Fixing the movie would actually be pretty simple.
1) Lose one of the villains: There was way too much going on and none of them had time to be properly developed. I think having Harry's goblin is a good way to end the trilogy so keep him in the mix but lose either Sandman or Venom.
Sandman is a cool character (Thomas Hayden Church did well with what he was given) and the special effects were great but having him revealed as the real Uncle Ben killer was stupid. I think ultimately you go with Venom and use a different actor than Topher Grace. Getting rid of Sandman would allow them to build up a better backstory for Eddie Brock.
2) The symbiote suit does not come off! This seriously bugged me. The scene in the church with him trying to tear off the suit was well done (just like in the animated series from the 90s) but doesn't really make sense when you having him take it on and off like his normal spidey outfit.
3) Keep the slowing getting evil/lose the emo hair and bar dance scene: Nuff said.
Do these things and it will be a better movie.