When I have a complaint about the third act of a comic book movie it's usually that the conflict within the third act is underwhelming. Only The Winter Soldier had truly great action for me so far in the Marvel movies before seeing Guardians of the Galaxy. The action in it was great in my opinion, and the battle didn't disappoint like most battles of recent blockbusters have for me.

Not that there has to be a LITERAL giant battle. For example, Dawn of the Planet of the Apes has an awesome fight at the end that is more satisfying than any big battle I've seen recently.

I just like to see big action in my comic book movies so when one fails to deliver said action it leaves a mark. I was personally disappointed by the finale of Days of Future Past, especially when cools stuff like Quicksilver's scene earlier had happened.

I think it's weird for someone to complain about GOTG having a battle in the third act, what the hell do you want from the third act of a sci-fi action movie? It's not like the characters became any less interesting during the third act.

Anyways, the third act is supposed to be when the biggest action beat happens in any story. Now when I say biggest it could be the biggest action taking the emotion and atmosphere into account, such that there's a big war type scene in a recent movie followed by a smaller but more emotionally engaging fight at the end.