So far, as far as I can tell, the world of online 3D model sharing is utterly morally bankrupt from a UX perspective. Well, from just about any perspective, actually.

Thingiverse doesn't actually let me to upload anything?

Pinshape doesn't have a built in 3D viewer?

Sketchfab had the model upside-down; doesn't let you rotate more than 180 up-down so there's no way to see the model right-side-up?

Shapways explodes saying the model is too big physically (not data wise); doesn't have any way to scale the model to the max allowed size automagically or manually?

3D Warehouse doesn't accept STL files?