Layout is almost WYSIWYG and easy to get started with a simple download, resulting binary is small, really easy to integrate with an underlying 2D or 3D scene,

Layout is almost WYSIWYG and easy to get started with a simple download, resulting binary is small, really easy to integrate with an underlying 2D or 3D scene, lots of controls, built in scripting for simple things, great interop with native code for complex things, can deploy to multiple platforms with one project, everything is open source.