SharpDX got it's toolkit, it's clone of xna but you can use dx11 features. I believe they've got their model loading working (not only .x and .fbx files, but many more).

I've been also using JBBRXG11: http://jbbrxg11.codeplex.com/ . It's something like add-on to xna to enable dx11 features. It also got animations for models.

Depending what you want to do, Bepu is my favourite 3D physics engine for c# (it supports XNA, sharpDX and slimDX), for 2d you got port of Box2D and Farseer

Yep, both MonoGame and SharpDX have integrated AssimpNet so you basically get all these formats...more or less. SharpDX even has the animation going, not sure about MonoGame.

Also, a shameless plug since AssimpNet is my wrapper project ;)

I guess I'll try that, any good tutorials for it?

You'll find that plenty of XNA tutorials work well with MonoGame, and since you pretty much need the windows phone SDK to use MonoGame (for content processing) the XNA specific tutorials should compile.

Unity 3D can be developed entirely in C# if you choose. In fact, it was created based on Mono/MonoDevelop but with many different options added. There are many free or low price art assets and even some world class third party products available. It is very cross-platform, running on the Mono Framework/Runtime. Everything the end user needs to run it can be included in the download, including the framework and runtime. The whole development environment is designed for the quickest workflow possible while allowing for extensible coding or libraries. Thousands of developers have completed games with it, some with big sales, so it must be a good game engine.