In one word : GREAT !

First, let's talk about the positive stuff.
I really don't know how you imagined the style but it is really really good ! Even better than the current metro ! It's like someone seing windows 9 and looking back into windows 8 : The first time you see windows 8, you can't find any negative point then when you look at this, you see the negative points. I had the same reaction when I saw aero in windows 8, I saw the negative point in aero in windows 7 (the close, maximize,... were not "fresh" enough).

The negative feedback, not much but yes :
The icons are not fresh enough, I feel like I'm in the desert (a bit exagerated) . You should add the names of the games. I rated 4 stars the originality because it's based on metro.

Conclusion :
Great work ! Just add some water on your icons !

In a word - Fantastic.

The execution of the design makes the whole menu look 'delicious' (if that's even possible). It makes you want physically touch the tiles and move them about. It made me imagine the subtle animations within the tiles.
THIS is what Microsoft should be aiming towards with their tablet UI implementation (and potentially desktop too).

The icons and tiles are soft on the eyes and the minimalistic approach to notification badges is great. They let you know that something is pending without being too 'in-your-face'.

The colouring behind the tile backgrounds seems a little arbitrary as there's no set colour for stock apps or at least for app categories (i.e. messaging apps to be one colour, control apps another etc). It makes it seem jumbled and disorganised.

Gaming icons are fine as they are (a mouseover to show titles would be good) however, some sort of overlay could be used to make them blend in with the rest of the tiles. they seem a bit loud and glaring. (Game developers may one day include tile images specifically for Win8 in the future to fit in with the UI).

Obviously, with any added tiles, such as the web links on the right, there's no way to make them fully fit in with the overall UI format. The way that you've shown it is one option, but perhaps another option whereby the background is the same for all and just the logo in the middle?

Shadows. At a first glance, the balance between shadows and tiles is great. There's a subtle feeling that each tile is hovering just above the surface.
However, when you start looking around a bit more and your eyes adjust, it brings about this effect [link] Obviously not to the same degree, but it has an effect on the eyes and slightly ruins the overall 'clean' look. Perhaps lowering the shadow radius will eliminate the problem, or increasing the spacing slightly between tiles?

The interior shadows on the tiles themselves (e.g. settings tile) are a bit overpowering when mixed with the reflection effect. Maybe reducing the radius again or making the colour of the shadow slightly lighter. Then again, maybe varying the strength of the shadow depending on the tile background colour.

The background wallpaper colour is a great choice. The soft grey with the outward gradient from center definitely keeps the focus on the tiles. Much better than the 'funky' designs and solid nature that comes shipped with Windows 8 Consumer Preview. Lovely choice.

Overall I think this is a great concept. Again, this is the sort of UI Microsoft should be attempting to create with Metro and, should there be any chance of customising the Start screen when the OS is shipped, you should definitely try and make this a reality. I'd definitely use it.

Great work