Here's version 2 of it now. Removed the black lines.
The remaining black lines are an error, they were supposed to be a darker version of the colour. I'll remove them shortly and update the image.
I'm likely going to leave the grass how it is but you are right about the windows. I just wasn't sure how to proceed with them.
Thank you for your kind comment :)
The shading seems to be rather inconsistant, or rather, the outlines. The outlines are a darker version of the colour inside at some points, and at others, it's simply a black outline. I'd suggest you make either a black outline for everything, or a darker version of the colour outline for everything. The windows could also use some work, they look like white rectangles as opposed to actual windows. Maybe you could make them a light blue, and add some shine?
One last thing, the grass texture looks a little weird. The green is too vibrant, it contrasts with the rest of the image's darker greyer tones. Other than that, looks pretty good.