JGame is an open source, multiplatform, JavaScript, FlashPlayer game engine for 2D games with sprites with automatic animation and collision detection, a tiles based background with sprite-tile interaction facilities, and more.
The simplicity of creating your own unique game using very basic coding skills (or even no coding at all). You can use any programming language you like to write code for it. It's cross platform so works in virtually every browser out there without modification. There are some bugs but they aren't really big ones - most people seem happy enough just playing around with what it has already got. I'm developing my first mobile app/game hybrid here.
Its not as polished or easy to work within asβ¦
Pros
The options for free are great and it's cross platform
Cons
I've read about lots of bugs and glitches but they seem to be very minor problems and most people use it for what it is and don't seem to mind