1) Tobey Maguire
He just wasn't a very good Peter Parker or Spiderman. He just seemed to be whiny and annoying. And god, that 'emo' phase from Spiderman 3. He just didn't play the part well at all.
2) The costume designs were pretty bad
This is mainly for the first one. I didn't like the Spiderman costume. I don't know why, I can't quite put my finger on it. Something just bugged me about it. As for the green goblin, that one is fucking awful. He looks like a villain from a really bad saturday morning cartoon. The green goblin is supposed to be terrifyingly scary and he just wasn't. The venom costume is just plain lazy. They've just turned the already bad spiderman costume black. Very lame.
3) Badly done villains
Like I said about the green goblin. It just isn't scary enough. He doesn't carry some of the most iconic weapons and gadgets from the comics. Dr. Octopus was probably the best, but still he wasn't russian. Hence taking out the whole of his backstory completely. Then the atrocious villains from Spiderman 3. None of theme were developed enough. Sandman was boring and badly acted. James Franco just wasn't right for the hob/new goblin and Venom didn't have enough time and sucked. I wasn't even sure that that was possible.
4) No Webshooters
This one really pissed me off. Raimi basically dismissed the fact that Peter was a genius and a science wiz. It just doesn't make sense why he would choose to do that. For me, he's taking a lot of character out of Peter Parker just by choosing to do that.
5) It's not funny enough
The best thing about Spiderman for me it's the humour of Spiderman compared to the nerdyness of Peter Parker. The trilogy didn't have that. Tobey Maguire played the same part for both of them, which shouldn't have happened. The whole point is that Peter Parker feels comfort in the suit and becomes confident, hence taunting the villains. This didn't happen, which was disappointing.