Amazing piece, really. I'm glad you put this up, i needed to write a critique, I haven't done so in ages. So I'm glad this came by!
Lets start of with:
The Overall image; The overall image looks really good, the stream of floating rocks blend in well with the rest of the picture, at first i thought they looked a bit out of place but when you download the image, it looks much better than in the thumb.
Colors; The colors ... They are in one word magnificent! The way the light spreads out trough the cloud gives off a natural feeling, also, the composition looks really good! Most wallpaper which are based on space have a repetitive pattern in them (like a random planet which is centered in the screen with some effects around it) This piece, however, has 2 focus-points. The Astro-cloud and the "Eye" of the cloud itself draw all the attention to them, well done!
Dynamic/flow; Like i stated before, the whole composition seems just right. The rocks dont look flat and neither do they look fake, like its copy/pasted on the image. The way they increase in size closer to the eye of the storm gives off that 3D feeling, the feeling they're rocks appearing from one end of the universe to another, again, well done!
Details; There isn't much to note about the details, I can congratulate you with the image quality however, many space images have too many details in them, making it look messy, this one has simple white stars in the background, which also blend in nicely with the rest of the image and cloud. And the way the cloud extends in the far left of the images also gives of a 3D effect, like its really a massive cloud. I can imagine it being light years in diameter.
Finishing Touch; I have one, little and really minor detail to note, about the title of the wallpaper itself. Its ... distracting, i think you should have kept in mind that most users with a 1920x1080 interface have a small task bar, what i do when putting my "Watermark" on an image is calculate that size. I just google:"Desktop Screenshot windows 7 1920x1080" or any other size, mask out the task bar and keep that area as my "copyright" area. So it wont appear on screen that much, but its still there. Its a great way to keep focus on the wallpaper itself, instead of a massive block in the center of the screen (Like with the planets)
Overall, I've rated this for Originality, Impact and Vision 5s. The Technique [copyright and links] could be improved by keeping in mind the task bar and how that works on different operating systems (Like, in W7 with 100% transparency it will blur the BG image, so thats great for putting your watermark in the blurred area so it wont be visible that much)
But again, this is only my technique and it doesn't mean you have to apply this as well!