The scriptability of Blender has been a big draw for me. It's pretty deeply integrated with Python. One can even use it as an interactive 3D front-end for a substantial Python script. That is, let the script create and manipulate the objects, and just use Blender to view what is happening. In Blender 2.4 it was possible to keep a script continuously animating the scene, something I found tough to make happen after the 2.5 overhaul.

Blender 2.4 also let scripts create UI components, such as entire control panels to influence the running script. The last time I checked in 2.5, this was either not possible or not well documented. But I'm not up to date on this.

I'm also not familiar for comparison with the scriptability of Maya or other tools.