- Durability. Home-printed blocks are likely to be a little rougher and a little weaker than the moulded ones from LEGO.
- Colour. The home-printed blocks will probably be all the same shade of milky-white.
- Creativity. As long as LEGO has designers working on interesting new models, they'll probably still be able to sell sets.
- Electronics. 3D printers will only be producing plastic, which means electronic sets like Mindstorms will still have the vital electronic controller pieces and wired bits that crafty bricksters need for their more advanced projects.
PS - They've already lost a court case about producing LEGO-compatible bricks.
PPS - I think the only illegal thing would be distributing trademarked designs.