Lots of potential with this image and scene. It looks like the image isn't quite level. A simple thing to correct but if its not corrected its distracting.
The conversion looks like a straight gray scale conversion or desaturation. My recommendation would be to use the channel mixer (if its available) to to create a more dramatic monochrome.
Good call on leaving out the sky. I would consider cropping off a bit more off the bottom leaving the reflection of the tree and the windmill.
If there was sufficient wind to get the windmill going a long exposure would definitely be in order. It would flatten the water and create a nice motion blur in the mill.
It would be nice if there was a third element in the scene. With the windmill and tree being the subjects in the scene it feels a bit static. Of course you could just wade across and cut down the tree too... ;)
Overall a pleasing image and one that certainly meets the challenge.
I know you can't do anything about them but the bits in the water are really distracting my eye from the subject. Maybe a different angle or a different composition that didn't include as much of the water. My only other little nitpick is that it looks slightly crooked.
You've got the rule of thirds going on here but, the bottom 2/3's is rather dull in the composition because it's lacking anything to look at. That means that 2/3's of the composition is of nothing of interest. I know that sounds harsh and I don't mean it to be, just giving a reason that it's not as striking a shot as it could be.