Duality is a plugin based 2D game development framework using C# and OpenTK. One of its main features is the WYSIWYG editor, which lets you playtest and edit your game seamlessly within the integrated sandbox.
I don't need to say something. The engine is perfect. The only thing it sometimes doesn't open Visual Studios 2013. But that's really the only thing.
Good luck with developing this good engine.
This game engine is stellar, in my opinion. It has a WYSIWYG editor, but that does not mean that the engine is "oversimplified"; this engine has just the right balance of code/design and I think it's one of the engines standout features. This game engine also features some sort of Z coordinate: the engine is completely 2D, but it can make use of a Z coordinate to signify focus or distance, and makes parallax effects trivial.
There is so much more I could say but I feel the best way to get to know this engine to actually use it. There are so many nifty features in it.
Oh, did I mention it's also open source?
Duality is what Unity should have done with 2d. Getting rid of the need to make 2d games in 3d is a huge plus. Not to mention it runs on directx11 in C#.
This is the best open source engine I've ever worked with. Clean and understandable code, all pure .Net, an incredibly fully featured editor, and an awesome component-oriented approach to building games are just some of the reasons I think deserves a 10. Have been using it for a year and a half and have had no major problems. Adam, the project owner, is also amazingly quick with fixes and updates.
Good all-around engine, with lots of customization options. Easy to learn while still allowing to work in-depth enough to develop everything you want.