This is a very strong cloudscape. The cloud patterns and the color variations make it work very nicely. I'm a bad choice of people to critique photos like this because I personally don't care for cloudscapes. My personal taste includes some amount of 'land' in the image to add perspective to the cloudscape. This is not saying that this is not a great shot because it is :)
I think your final score here definitely supports the overall opinion of the site... 6.4 is a great finish...
I love your sky, but I think if you'd had a little something from the land in the photo, the picture would have seemed a bit more solid. Definitely a cool sky, though.
This is an awesome sky, but I would have liked it more if I had been able to see the land as well. You must have had a good reason to crop it out. Almost a perfect shot.