Phase 2 has generally been a step up from Phase 1, TWS & GOTG in particular, but Avengers is really solid. The shot composition (for the most part) and some inspired camera moves are what elevates it. The only issue to me, moreso than the aspect ratio, is the flat lighting that particularly stands out in the opening and on the Helicarrier (which is how a Helicarrier would be lit I guess, but whatever). It doesn't really pop for whatever reason, despite some strong contrast here and there. I know Whedon wanted this to be a war movie, and hired Seamus McGarvey as DP intentionally for this reason, but I think the infusion of comic book color got cross-wired and there were muddied results. Whether this falls more on Whedon or McGarvey, both seasoned pros, we won't know. However, in the commentary Whedon says after shooting the great-looking Cap punching bag scene, he pretty much surrendered lighting duties over to McGarvey.
So it's a tad mixed for me, but I lean much more to the positive end for showing some voice & ambition in framing.