Xbox360 worlds are limited to 4096 chunks. If you find every single diamond in the game you'd have about 14,000 diamonds, depending on how large the veins were. That would take quite a long time though, of course.
Well, I think you should work on your outside structures. Especially that dull cobblestone tower. Add some materials to it, or don't have it going straight up. Think of a nice pattern.
Also, try to build into the terrain instead of making it flat for your buildings. Things look better when they look natural.
Honest rate: 7.5 (Coming from a critic)
But it all has a lot of potential. You have the basics down for everything, now you just need to expand by thinking outside of the box