Too much crammed into a 100 minute film. It has six action sequences, and it technically isn't even an action film. That should tell you everything.
The fights are good, yes. But there's little time to develop the villains as being any meaningful threat, so potential emotional connection is thrown out of the window, instead favouring a very pretty but shallow aesthetic. Yet at the same time the fights distract from the central romance that is key to the plot, hell, to the entire film. So Scott Pilgrim loves Ramona Flowers, why? He seems to have only known her for about 3 days, and two thirds of the time he spends with her is devoted to fighting off the exes. There's no depth to the relationship as there simply isn't time to concentrate on it whilst simulataneously delivering some vivid smackdowns and tonnes of geek references.
But then Scott is also in a band, and has a previous girlfriend, and another before that. So there's just more and more being stacked onto the pile, and nothing gets truly explored. The cast is strong, but none of the characters get much room to expand, instead remaining one-note (obsessive ex, cynical band member, gay roomate, list goes on). Really speaking it's ill-suited to film, a one-off telly series would have allowed for some character development and narrative growth. Or better yet, change one teensy little thing:
Instead of Scott defeating the exes in order to continue dating Ramona, simply have him defeat them in order to ask her out. There, boom, all lack of depth to the character of Ramona is dealt with, and the lackluster ending can be replaced with soemthing far more apt.
Looks cool though.