Well, the live-action actors in the shot aren't lit quite the same as the other elements--the backlight specifically doesn't match the other objects. Something about the shadows looks off too,like the actors were originally lit differently and the dark areas were curved up for brightness. Also, if you look at the soldier to the right of center standing on his own behind the fern frond, there's a big black fringe along the back of his arm due to motion blur. To a lesser extent, the same issue is visible along the limbs of several other soldiers.
The power armor doesn't look right in part because the smooth, flat components of the arms, hands and guns have that too-even shading problem that tends to crop up on very flat or smooth CG surfaces. I don't know if there's a technical name for it, but I've always thought of that as a "glowy" look, where the lighting illuminates a surface too evenly and almost makes it look incandescent rather than being lit by an external source. The metal's highlights look like they're being accomplished with conventional specular shading rather than accurate raytraced blurry reflections that have become all but standard these days – that's TV-level or 1999-era movie caliber work.
I also take issue with the power armor's texturing; it too is very even and doesn't seem to interact with the mesh at all--it reads to my eye as though the metal was all textured with a generic "damaged metal" material that isn't taking edge wear, grease, leaks and damage form the associated moving parts into account. I'm more willing to cut the CG team some slack on that as they seem to be trying to match the physical props, which look pretty phony plastic too. But I still don't like it.
And since I've been employed doing 3D in the past and will be moving to LA this January to intern at
Zoic Studios, I like to think my opinion carries some weight.
