I never really understood the appeal of programs like this. It's not hard to implement achievements in your game, so what's the point? Another conceptual problem with this is that this is kind of a "standardization" of games which I don't like. It limits what games can be.
This idea is obviously copied from Steam but I think not every idea from the proprietary world is worth copying.
Anyway. As for the program.
And the screenshots look kind of ... ugly? It looks too technical and not visually appealing, kind of disappointing tbh. You're not going to attract users with ugly screenshots like that.
Also, I agree that .NET is a bad idea. .NET is not really great for FOSS. To be able to use .NET programs, you need Mono, and Mono sucks. The problem with .NET is basically that the official Microsoft implementation has more features than the specification, so Mono is kinda doomed to suck. Classic EEE bullshit. .NET might be technically compatible with free software, but it still sucks. FOSS programs should generally shy away from .NET.