Features

Doesn't take over your application when it comes to initialization of external libraries.
Clean modular object oriented design.
Graphics support for SDL, Allegro and OpenGL.
Input support for SDL and Allegro.
No themes! As with Java's swing, member functions overloading and inheritance is the way to go.
Comes with a set of standard widgets.
No threading! The user of Guichan always has the power to tell Guichan when it can perform logic and when it can draw itself.
Licensed under the BSD license and therefore static linkage with Guichan is allowed.
Very small compared to other GUI libraries like Qt or GTK.

Missing features