Well the emo dancing was the nail in the coffin for that movie.
See what's funny is that Spidey 3 has some of problems you mentioned as a negative for ASM 2. Convenient amnesia, convenient connection of Sandman to Peter, the lame random team-up of Venom and Sandman at the end. Nothing flowed naturally in that film. Furthermore, they made Mary Jane into this naggy, needy, bitch. Instead of feeling sympathy for her and her career nose-dive, I felt as if Peter should just leave her.
Goblin was the biggest letdown, as Harry had 2 movies to establish himself and have him get a major arc in 3. Instead, they just threw him in the beginning as a way to get the "Harry becomes Goblin" out the way, and they didn't even get Goblin right at all!
Venom was the best part of the movie, and that's because they screwed him up the least.
As far as the nerdy Peter, the director said this back when the first ASM came out: