I'm not against customization (I love my set of wallpapers!), but I'm just speaking about Metro. It works best when it looks simple and minimalistic. Throwing lots of patterns and colors at it just makes it look clumsy.
Not to mention iconic.
If Microsoft wants to start eating into Apple, they really need to present something bold, uniform, and elegant. Plain Metro is that. But a lot of those designs are just plain messy.