A very cool idea with impressive results, but sadly, side-scrollers will only have left/right sprites, and top-down games will only have front/back/left/right. Whereas the Mario example in the article also has top/bottom.

So to actually use this, you're going to have to be able to make the missing sides yourself. And if you can do that, you can probably design a better 3D block model as well.

Not necessarily. 3D modelling and sprite artistry are very different skills.

Not so. 90s FPS games (such as Doom) use enemy sprites with views from multiple angles for each frame. Would be a good test project for this and would dovetail nicely with the existing Doom voxel project.

Related: http://voxelbuilder.com/ - supports building your own models and either printing them to build paper models, or generating STL files.