1st of all, having a 'real' physics engine in blender (with constraints and all) is AWESOME! But as i was making stuff, and looking at demos, i noticed that the friction between objects is nearly infinite, and the friction settings in the material dose not change it. If you just haven't done that yet, i understand, but it is kind of annoying...
keep up the good work.
The friction is lower if the mass is higher. Or, a better way to say it is that friction dissipates x energy per second, so the more mass the more sliding. This is unrealistic, in real life the mass = the energy dissipation. So make stuff heavier to lower friction.