My belief is it would have been better if either the Venom or Sandman story dropped. Harry's story was necessary for this movie to wrap up the trilogy, that was a given, but the inclusion of both characters was at both's expense and each was criminally undeveloped and sloppily written as a result.
What was the mystery illness and the cure for Sandman's daughter? What was that experiment trying to accomplish and why were they performing it in the middle of the night? How did Eddie wind up in the same church out of the thousands of churches in NYC? Why doesn't Eddie question the black ooze that just took over his body and just accept that he has all of Spidey's powers? Why does he immediately team up with Sandman, didn't he always want the pleasure of killing Spidey?
Venom I thought was the most shortchanged, I, admittedly, was never a big Sandman fan but I thought he was well realized visually (didn't like the whole flying sandstorm thing though) if not as a character. His origin was pure 50s B movie in a bad way. Venom should have had a scene where he explained to Eddie why "they" were Venom. The film missed the opportunity to show Spiderman against a villian invisible to his spider sense. Instead of Peter spinning around and being grabbed from above, imagine after a prolonged chase and Spiderman moves into a dark room, up in the corner of the ceiling Venom's white eyes opening in the darkness. While I'm ranting, how about this scene ripped from the comics, where he comes to Aunt May's house as Eddie and asks for Peter, they then walk down the street and Eddie explains how this was personal and he wouldn't hurt people like Aunt May, "innocents"? Visually he looked the part but I'm afraid he was VINO, in my humble opinion.
Ok, I feel better.
Ultimately I think that they should have introduced the symbiote, developed the Sandman, and finished the Goblin storyline. In Spidey 4 you could have introduced and developed Venom properly. Oh well, too late.
My $.02.
IMD