3 things killed SM3 for me.
-Trying to do too many villains. The first two movies worked great just focusing on one villain, why would you screw that up and try and do two (hell, 3 if you include Harry Osborne)?
-Venom (specifically Eddie Brock Venom) has always been my favorite comic book character, so for them to take Eddie Brock, a guy who's 6'4 and built like an NFL Linebacker, and have him be played by Topher Frickin' Grace, a guy who looks like he weighs 135 pounds wearing concrete shoes, was just utterly bizarre (and yes I know there's "Ultimate Marvel" and all these other variations, but I like the original Venom). It also killed it because part of the whole Spiderman vs. Venom dynamic is that Eddie and Peter are total opposites: Eddie is a jock, Peter is a nerd, Eddie doesn't take shit from anyone, Peter is a doormat, Eddie is highly emotional, Peter represses his emotions, etc.
-Goes without saying, but the sequence of "cool Peter Parker" under the influence of the symbiote is just embarrassing. It's one of those scenes where you almost feel embarrassed watching the movie. Why couldn't they just focus on him becoming more violent and be done with it? I get the whole "well he's a nerd so he thinks it's cool", but most nerds I've met think being a bad-ass is far cooler than dancing around in the street, and thus logically, it should focus more on Peter abusing his powers by pushing others around.