The mistake this movie, and many comic movies make, is cramming too many villains into one 2 hour flick. Harry or Sandman should have been the main villain in this film, preferably Harry since they've been building up to him this whole time. Harry's after Spiderman; Spiderman doesn't want to kill Harry; Spiderman gets symbiote; Spiderman loses his humanity and becomes merciless in life; It drives a wedge between him and the people he loves; Spiderman torments and ruins Eddie Brock's life; Spiderman kills Harry; Spiderman realizes what a monster he's become; Spiderman removes symbiote; Eddie gets Symbiote; end of movie. Bam! One villain, one villain for the fourth movie to increase anticipation, no need for motive building and you can get right into the conflict for the fourth movie. Venom really had no reason to be in this movie other than to appease the few people clamoring for him. Although I liked Sandman, the movie could have really done without him as well (though the CG effects surrounding him were awesome).
Like people before me said, the plot was akward at best, but it was tolerable enough so that the special effects made up for it. Overall ok.