I like your original take on the "circle" idea. I would have given you a high score because I think your entry is creative. That said, I think a lot of people might dismiss the idea. Since the average voter (including me, depending on the challenge) might only spend a limited amount of time on each picture (probably a couple of seconds!), an image that doesn't immediately connect with the challenge topic might get voted down a lot.
I read somewhere that voters here tend to prefer images that are simple over images that are busy, because when you are flipping through hundreds of pictures, you are going to stop only at the eye-catchers. So if the focal point of interest is not immediately obvious, voters might use their 4s and 5s to vote "next!" The darkened leaves by the trunk and by the are also distracting -- the overall composition might flow better without them.
Great looking shot and well composed. Although your blacks are perfect, your whites are not as perfect. They're not white at all but grayish. Maybe this played in your low score. I'm more in tune with the idea that you were stretching it a little bit too much and people didn't see the roundness in the highlighted sky. I keep going back to it, which is good. :)
I really like this. It's beautiful. You've captured several different levels of light and maintained detail without overblowing the highlights or losing detail in the shadows. Really really nice.