Somewhat biased suggestion for a framework to take a look at: Duality. I'm currently working on a "Getting Started" kind of tutorial that might give you a quick overview on how to use it. You'll find it in the Wiki section of the google code page I've linked above.
Pro: Extensible Game Engine, Visual Editor, specialized on 2D, based on OpenGL / OpenAL, designed for C#, completely Open Source.
Con: Still under development, relatively new (been around for 1-2 years), Windows only
There are other 2D game frameworks out there for C#, though. Probably plenty of them - or so I imagine. SFML has been mentioned and you seem to have already found MonoGame. You might want to take a look at this list, although C# isn't heavily represented there. Here's another one.