I disagree on peoples complaints with IM2 but that is neither here nor there.
I've read several critics who think it works fine on it's own and others who think it's too Avengers crowded. I think it's just a hollow point of complaint. An easy target when one doesn't want to look at the story but yet get invested on the sidebar.
Captain America is working it's way up the RT score and was at 54% last I looked, up from 33% off the bat this morning. Metacritic was 59% not too long ago.