It's not that designers cannot round corners or simulate woodgrain to their heart's content. It's just that it isn't necessary for developing software which fits within the overall design aesthetic...a designer can even use serifed fonts if they wish, it will just be more work than using the standard and probably to little benefit for the user.
Where Metro excels is in allowing the use of branding to communicate with users - e.g. Twitter and Facebook applications are recognizable by their logos - and Metro's simple aesthetic favors designers who can develop similar iconography for their products.