two words: Character derailment
The fact that Gwen Stacy slept with Norman Osborn just ruins this entire storyline for me. Not only did she cheat on Peter, but she cheated on him with a 40-50 year old man. Did Norman really need anymore motivation to kill Gwen other than to piss Peter off? Let's not forget that she's friends with Harry Osborn, meaning that she slept with her friend's father!