Everyone else is mostly using Java or Flash or
something specific to iOS or Android.
That's not true, there's a large ecosystem of game developers writing in managed C#, in the form of the Xbox Live Indie Game market. It shouldn't be difficult to port those to Metro, assuming Microsoft doesn't shoot themselves in the foot somehow.
The quality is debatable, but there are over 2,500 indie games according to http://marketplace.xbox.com/en-US/Games/XboxIndieGames. I've heard some success stories.