Torque 2D is the world's most powerful and easy-to-use 2D game engine. Built atop the common Torque core architecture, it offers many of the features of our cutting-edge 3D game engine, but customized for 2D gameplay. With development paths that allow publishing to Windows, Mac, Xbox 360*, Wii*, iPhone*, your game will reach the widest possible audience. And with its intuitive and powerful editor, anyone can jump into game creation with little to no prior knowledge.
TGB is a Solid piece of middlware with lots of functionality and a very robust scripting language that lets you create the majority of your game without having to break out a C++ compiler.
The basic level editor and drag and drop functionality of images, particles, tilemaps etc makes rapid prototyping a breeze and blast.
However that'll only get you so far. You will need to write lots of script, and the torquescript is a pretty easy to use language as far as that sort of thing goes.. about on par with LUA. only it's its own beast and full of little oddities and quirks but the documentation and reference that comes with it is pretty good and the forums and resources parts of the website are quite handy.
The Torsion editor (standalone 3rd party product $35) is essential for debugging.
[edit: 1.7.6 came out today. so my 1.7.5 version specific complaints have been removed and the extra point awarded for GarageGames fixing those critial bugs]