I'm writing from the position of a huge Spider-Man fan, also, from the position of someone who liked both of the previous films, so yeah, i'd say a big part of the crappines in this movie is the huge disappointment i felt after watching it.
Neither Daredevil nor Hulk managed to make me feel so bad after watching them, and both were pretty much z-grade movies with half-decent special effects. :/
Yeah, Sandman wasn't bad, i'd say him and Harry were the highlights of Spider-Man 3. Not really redeeming, by any measure, just makes them look out of context when put side by side with horrific abominations like Venom.
About Carnage - he isn't THAT bad, really, and certainly is better than every single other symbiote that was introduced later on (MAYBE except for Toxin, but they made him look like crap). I always felt there's a whole darn lot of untapped potential in Carnage as a character.