You know, when I think about it...Spider-Man 3 was destined to be a lot different from the first two movies no matter what. The first two movies each focused on one villain...it's a hell of a jump to go from one to three. I mean, even if it had turned out good, I think it would have still felt a lot different than the first two because of that.
The thing that bugs me is Venom just didn't belong in the Raimi-verse. If Sony had just let him make the film he wanted to make, we could have had a nice "Silver Age" trilogy. I mean, I get why Sony and Arad pushed him into it...the franchise was huge and Venom is one of Spidey's most popular villains. But the moment Arad stuck his head into it, it was doomed to be a case of too many cooks in the kitchen. I can't imagine what a nightmare writing that screenplay must have been, with characters popping in and out through each draft.
Imagine how much more excitement there'd be towards the ASM franchise if Venom hadn't been done yet and they were able to introduce Brock earlier on to give the symbiote saga some room to breath. Then again, if it weren't for Venom who knows if a reboot would've happened?