Venom isn't bad in concept. Distill it down and he's a bully. Well, the concept is bad. But the moments where Venom attacks Spider-Man's vulnerabilities are entertaining. The Spectacular Spider-Man cartoon handled it really well, down to how he's defeated. I agree that his popularity is because he's the 'bad-ass spider-man.' Also, his design increasingly became a stupid pile of muscles, teeth, tongues, ooze, and shit.
Speaking of shit, there isn't a defense for Carnage. Seeing the snes game at west coast video and the comic covers made him look cool. Then you read it and he's just awful. I have a comic post-maximum carnage where he breaks free by biting his lip, and there's this "not this shit again" feeling. Spider-Man shows up with a sonic blaster that's destroyed. And Carnage leaves...for some reason. They try and flesh out Kasady with stories of his youth. Eventually he confronts one of the person he knew as a kid and says something like "I DON'T NEED THE SYMBIOTE TO BEAT YOU." And when he reverts to human, Spider-Man comes out of nowhere and punches him in the face, knocking him out cold. OK.