There's only drag, bounce (restitution) and friction. I think it would be useful to have a mass (weight) property too.
I know there's only density in box2d, but I think mass would be more understandable for flowlab game creators:
Mass = density * area size
That means when the block gets 4 times bigger, you'll have to make the density 4 times smaller so the mass keeps the same. The mass will automatically be adjusted when you change the object's size. So only advanced game creators will change it.
So.. What do you think? Mass or density?