My big disappointment was with just how lazy and clumsy this film was put together. I don't think it's a bad one, but certainly not even close to what Raimi and the team showed us they were capable of with Spider-Man 2.
First off, the whole Harry has amnesia business was just silly. Heck, I don't even understand why it was necessary at all. If anything, it gave James Franco the chance to act kinda goofy and ham it up. I also didn't understand how Harry got MJ to break up with Peter. He kinda just popped up, scared her, and she went along with it. He wasn't forcing her into anything. You'd think MJ would at least warn Peter about Harry, but nope. She just blindly agrees to go through with it.
Now we arrive at Peter turning into a jazz dancing, gothic a-hole. The whole sequence was pretty bad, but in an oddly funny way. Personally, though, I think the movie would've been stronger without the Jazz club scene where Peter's dancing with Gwen. And a little more subtlety may have helped out the characters more than Raimi's more goofy portrayal of Peter's dark side.
Sandman was just very underdeveloped. He's got a daughter whose sick. This is what drives him. Besides that and the fact that he feels guilty for killing Ben, he's really got nothing going. He robs banks. Presumably, this is for money to save his daughter, but to what end? He just shows up, causes a mess, and then he's thwarted. What's the plan? How's he gonna use the money to help his daughter?
With Venom, I think they could've done better. He really didn't seem that threatening to me. Maybe it was Topher Grace's voice coming out of the thing, but the character was lacking in presence, even when he was fully suited up.
Overall, the movie felt sloppy. Everything just seems so convenient. It doesn't evolve naturally. Things happen purely because the script says they do. For example, Sandman just so happens to fall into a research facility and get demolecularized or whatever. How convenient! Also, the Sandman-Venom team up was just so poorly handled. Sandman's just walking down an alley when Venom pops up and says, "With our powers combined, Spider-Man can never stop us!" And then they go about their plan, which is told in a similarly sloppy and forced manner. Just way too rushed.
The Harry-Spidey team up also felt quite contrived. We all knew it was going to happen, but they could've made it feel more natural. And what was up with the Osborn houseman stepping up to tell Harry that that stuff just when the movie needed him to, rather than during Spidey 2 when the same character criticizes Harry for his unhealthy obsession over Spidey? Anyway, when they do team up, it plays out more like something you'd expect to find in Hot Fuzz.
I think audiences have come to expect a certain level of quality with the Spider-Man films. So I think all the anger comes from the thought that Spider-Man was supposed to be great, but turned out to be mediocre at best.