Why does Spider-Man 3 suck so very, very much?

First off let me say unequivocally that, yes, Spider-Man 3 does suck. It sucks mightily. It sucks harder than a Hoover. It sucks like a very expensive...never mind. I don't care if you're a fan boy, I don't care if you actually like comic book movies. If you can't see how horrible this movie is then you're obviously living inside a schizophrenic delusion, and more power to you. It's bad. And the reason it's bad boils down to one simple thing:

Writing.

The writing is god-awful.

To be even more specific, it isn't the dialog or the setting or the themes, it's simply the plot. Or lack thereof (yes there are cinematic and directorial issues too, aplenty, but I think the movie could have survived those, assuming that it had a halfway decent plotline - which it doesn't).

I will now proceed to point out specific instances of bad plotting in this movie, for illustrative purposes only. If you don't want to know what happens, and are actually waiting to shell out money for this travesty, by all means don't read what follows.

1) Deus ex machina madness
- Symbiote from beyond the moon: This thing comes flying out of space, for no reason at all, and it just happens to land by the one superpowered guy in the whole world? Just accidentally? Seriously, there is such a thing as too much serendipity. Even the original comic book had a better explanation, and that was basically that it was a gift from a god. Who liked to fight.

- Sandman creation: Whoops, I slipped on a fence and now I have superpowers. Who just leaves a particle accelerator sitting around in New Jersey? And what the hell were they trying to do with all that sand anyway? They weren't teleporting it, they weren't disintegrating it...apparently they were just trying to make a next generation concrete mixer.

- Harry's amnesia: Now I love daytime soaps as much as the next straight man, but geek-boy please. Amnesia? Bump on the head and I convieniantly forgot everything up to the point where I want to kill my best friend. And remember, it's not like Harry was exactly a friendly happy guy before hand. Plus, shouldn't he think he's still boinking Mary Jane? Hmmm?

- Post Hoc Butlery: This is the least excusable thing in the whole damn movie. You had to come up with a crappy way for a homicidal maniac to turn into a heroic do-gooder at the last moment, so you just put in a butler who apparently witnessed the aftermath of the fight between the original Goblin and Spidey? Apparently this previously non-existant butler was also a CSI with experience in self-inflicted glider wounds, and a side degree in the psychology of self-destructive nutballs. Two words - BULL. SHIT.

2) Melodrama isn't just a river in Egypt and neither is cliche
- Cry me a river: Is there any major character in this movie who isn't inordinately lacrimose (I always wanted to use lacrimose in a sentence)? Freakin' waterworks in here. As if that's the only way to express angst, sadness or generally emotion. I have no problem with tears but this is enough saline to hydrate an entire hospital ward.

- Real pals jump in front of spikes for each other: Is there any more cliched way to show redemption? "I'd take a contrived plot point for you, man!" And he gets impaled and dies by his own glider too; hoisted by his own retard you could say. If you were a dork, I mean.

3) Character development what now?
- Mary Jane is a whiny idiot: No other way to say that. I understand that they were trying to make her sympathetic and Spidey selfish, which would have been totally true, and totally cool if handled even remotely well. But it wasn't. She doesn't tell him that she was fired, from headlining a BROADWAY SHOW, for like what? 3 months or something? And THEN she gets pissed when he doesn't know. Not only that, she ignores him when he's actually being insensitive, but then gets pissed when he tries to commiserate over the cost of fame. Total disservice to her character.

- Spidey good/bad/good: You can actually figure this out from the opening credits. And they're only made from CGI webbing. Honestly. No rhyme nor reason required.

- Eddie Brock the psycho killer: We meet this guy, who is basically a self-involved jerk, but doesn't exactly seem evil. Then for no real reason he decides to kill Peter Parker, which he then happens to pray about, in the same church where Peter Parker drops the symbiote. Ug. He doesn't have anywhere near the kind of pathos or angst or genuine evil to make him a convincing, or hated, bad guy. He's just tossed in at the last minute to satisfy fan boys and CGI nuts.

- Sandman's daughter: Okay, so he's not really a bad guy - I get that. But why, oh why, does a man made of sand need to kick open a bank vault or an armored truck? Couldn't he just, I don't know, turn into sand and go through the air duct then get out with the loot the same way? Spidey can't stop what he doesn't know is happening. Plus, and this really chafes my hide, what the hell happened with his daughter? They just leave that thread hanging! And for my money, I bet that even if there is a sequel, which I hope to god there won't be, they never answer that question.

Alright, so that's my rant. Now I turn it over to you - agree or disagree? And more to the point I'd love to hear some writers make suggestions about how this horrific plot could be mended. If it was up to you, what would you have done? How could this have been better written?