ehhh..... while MR and a couple of the others are accounted as better render environments (usually), there probably wouldn't be too much difference here. Try toning down the 'quality' level on mental ray, and see what happens.
Your stuff's not looking bad, but it does smack of a computer rendering (although anybody who does this regularily should twitch to this more than one who doesn't). Mostly I blame your texturing, which, really, is on the simple side. I can reccommend a couple of things that shouldn't take too long, but will improve the apparent quality of the shot (and make it more 'realisic' to boot).

First, give everything a bump map (at least a procedural one). It doesn't have to be very big or very obvious, but you'd be surprised what even a small one will do.
Second, don't have ANY of the textures as one color. While the bump map will also go a long way to changing this, by simply putting in small patches of color on objects you'll make things look a bit better. Okay, yes, there are times when things probably should be the same color, but this probably isn't it.
Third, make sure nothing is just a simple plane. That is, make sure the roof has thickness. And while we're on it, try seeing if the ridges on the roof can either be actually modeled, or at least displacement or really heavily bump-mapped. Currently, that roof piece looks really flat....

That's all that really comes to mind at the moment. Try those changes, then show them again. Your shadows appear good, and I'm glad the windows have a reflection (maybe do something to make there be a single 'highlight' point or streak or something, along with the general reflection? dunno). Although, it kinda looks like maybe your light is white? Maybe make it a little yellower or something, and try downing the intensity just a little.

But yeah, not bad. It's really pretty good, in most ways.