Metro designs require imagination but provide endless possibilites.

I've heard it claimed that due to the restrictions that are imposed on Metro style apps "they're all going to end up looking the same" and "there's no opportunity to differentiate".

I'm calling BS on this.

Not only are people making such claims forgetting about the myriad applications which were all battleship grey and/or seemed to want to copy Outlook regardless of whether it made sense or not.
Such claims are focusing on the restrictions and ignoring the opportunities. Metro is intended to "inspire innovation".

Let's consider some other design opportunities which may be considered restrictive:
Watches are simple things. They have a strap and a face. If digital they'll show some numbers. If analog they'll have two or three hands. In theory that's very limited, but there are thousands of different looking watches available.

Or consider shoes. They have an upper, a sole, a heel and possibly some kind of fastening. Again, that's a very simple set of components but again, there are thousands and thousands of shoes available and they can look wildly different from each other.

Within an application we have many more options and possibilities when compared with something simple like a watch or a shoe.

If you find apps are starting to look the same then it shows a lack of imagination and creativity by the person(s) creating the app. It's not a limitation of the platform. It's a limitation in the imagination of the people creating them.