You know people are kind of running out of ideas when they start doing serious "retcon" in the third installment of a series. For the non-comics geeks out there, retcon, or "retroactive continuity" is the adding of new information to "historical" material, or deliberately changing previously established facts in a work of serial fiction. When I initially hear that Sandman/Flint Marko was going have been responsible for Uncle Ben's death, i thought, "OK, they are giving the nameless thug a name, and that name will be Flint Marko, and he'll become Sandman." Not unreasonable in order to give Spider-Man the vengance motive that is such a large theme in the movie. (The first "BatMan" film did a similar thing, makeing the crook who murdered Bruce Wayne's parents turn out to be the man who becomes The Joker: a large deviation from the comic book continuity, but the movies are their own continuity, since entirely retconned by "Batman Begins.") Instead, we see in flashbacks different versions of the killing where marko is present, all of which are different from the one WE all saw on screen in the first film: BO-GUS, in my opinon.
There are a number of plot flaws, one extremely major: The goblin-potion driven Harry attacks Mary Jane and coerces her into breaking off her already rocky relationship with Peter, which she meekly does. This is TOTALLY out of character for the Mary Jane Watson of the comics universe and with what we have seen of her so far. NO WAY she would have done it without trying to tip Peter off eithr during or after, especially since she has seen that Harry has superpowers and is off his rocker, but that doesn't happen.
A couple of less important quibbles: MJ gets fired from her Broadway gig due to critical complaints that her "weak voice doesn't carry past the first row." If that were true, don't you think the producers would have noticed that before opening night?
In their knock-down drag-out brawl, Peter ends the fight by deflecting one of Harry's pumpkin bombs back at him. Harry survives the explosion, but we see him later with scarring to the side of his face. Later, a similar bomb entirely vaporizes Eddie Brock and the alien symbiote. Perhaps Pumpkin bombs come in different yields?
And speaking of Harry's scarred face: It's noted that the head injury he receives in his first battle with Spider-Man heals rapidly and leaves no scar. The blast from the Pumpkin bomb injures the right side of his face, which we see healed with unnatural rapidity, but scarred. Perhaps healing a deeper injury takes longer for the scarring to go away?
One of the crisis points is when Peter tears off the alien "suit" in the church bell tower. So, how'd he get home naked and with no web-shooters? (In the comics, Peter already knew the alien was vulnerable to sonics, and so was prepared--.)
Cool points: Whatever it's made of for real, the black Spider-Man suit is a lot sexier than the red-and-blue leotards--.
The airborne battles involving Harry and the Goblin Gear are really exciting, done with great speed and bravura. The combo of Harry and Peter fighting together against Venom and Sandman was bot the coolest thing in the film and the most heart-wrenching, since you know that, one way or the other, it isn't going to last.
There were dependable and entertaining performances by the regular cast, with McGuire doing as best he could to overcome some rather lame writing of his character under the malign influence of the alien. James Franco stood out for the character changes Harry undergoes, and the aforementioned Thomas Hayden Church as the luckless Sandman. I didn't much care for Topher Grace as Brock: it's a a very different characterization than the comics, but I guess it works, more or less--. The comics version of Brock was already a thouroughly bad and dangerous man before becoming Venom, instead of a merely venal jerk, so that his movie transformation into a possibly cannibalistic monster is not well set up.
"Spider-Man 3" set new box-office records opening weekend, so I suppose potential is there for yet another entry in the series, although I'd have a hard time getting excited about it. We've already seen most of the best Spidey villains now. Although we've seen Kurt Connors, who becomes The Lizard, and John Jameson, who becomes "Man-Wolf", neither of those are particularly exciting villains. Perhaps a better choice for next main villain would be an updated "Mysterio," just because he's very different. Only time will tell.