After fiddling with snow in blender, and studying the original photograph I think I’ve figured out a few things which are making your scene look odd, especially compared to the photograph.
The first thing I noticed is that the shadows from your main light source are much too strong. Overcast illumination wouldn’t produce this kind of contrast. You might be able to get a better result by using an area light, or you can fake it by using point or spotlights arranged in a grid, just be sure you spread them out enough. You could use yafray’s Global Illumination, but that tends to be grainy, or take forever to render.
I had two observations about the snow. One was that the snow heaped on the two stones is much lumpier than the snow on the roof. You could make the snow on the roof more lumpy, but that isn’t consistent with the photograph. Instead it probably makes more sense to make the snow on the stones flatter. Also the snow under the bridge, since it was blown there should have subtle ridges and other irregularities.
The other thing I noticed about the snow is probably a big part of why your scene doesn’t look like the photograph. Since the snow on the roof is illuminated by the sky, the brightness of the snow and the brightness of the sky should match. So much so, that if you took out the trees behind the bridge you wouldn’t be able to tell where the roof of the bridge ended and the sky began. If you look at the photograph again I think you’ll see what I mean.
I messed around with snow textures in blender and since snow crystals are really small you want very fine grained noise. What I did was use a cloud texture, but I set the noise type to cell. This gives really silly looking results at first, but if you set the texture sizes in the map input material buttons to something like 100 or more. It will start to make a much finer grain. also since the light from the sky not only bounces off the snow, but through it. You might find that adding some emit to your material makes it look a bit more realistic.
The bricks you have are different from the photograph in a couple of ways. The first being that the ones in the photograph are darker and don’t have white between them. Also the reflection is darker in the photo, the only thing you can really see is the outline of the bridge against the sky. What your bricks could really use is a reflection map, so they’re more reflective on top and less reflective in the cracks. A bump map would also add to the realism of the bricks.
I hope at least some of this helps. I know it can be hard to spot some of this stuff, especially after you’ve stared at the same picture for a long time.
edit: I know there’s a lot here, so if you’re confused just send me a note and I’ll try to explain stuff better.
