I like your composition, though it appears to me as though the horizon could use some straightening. At first sight it seems like and overcast day but the clouds seem to say otherwise. You probably should have exposed for the trees to get more detail, I don't think that would have blown the grass in the foreground, and the sky would still have been fine.
You sure your monitor is correctly calibrated? Mine is and the top half of the shot is so dark there is hardly any texture visible, at least in the trees and most of the sky but the bottom half is nice. Anyway, not to my taste cause of the dark top, shame as it could have been.
The field is indeed a wonderful green colour. I find the blackness of the sky and the tree line far too overpowering for my taste and feel it takes impact away from the overall presentation of this shot.
Polarizers are great (and, in this case, really make the clouds stand out) but keep in mind what they do to exposure. IMHO, this image is just a tad dark.