I'm starting this because while we all know, or at least many of us, that Canon remade their mount in the '80's we don't really consider some of the technical ramifications of that change.
First up, I'm sure most of us have realized that Canon made their new mount much larger. This allows Canon to be able to design their lenses in ways others can't, and I believe this may be why Canon's 70-200 lens doesn't have the focus breathing problem that other manufactures' 70-200 lenses have. I could be wrong, so it might bear some research, and this also might mean that the other companies are out of luck making a truly comparable lens.
Next, why does Canon not let you put EF-S lenses on full frame bodies? I'm pretty sure that I read in some of Canon's documentation, somewhere, that the reason for this is that for at least some of their EF-S lenses, they place the back element further back in the mount to place the lens closer to the sensor. In an APS-C camera, the smaller mirror won't hit the lens, but if placed on a full frame which has a larger mirror, the mirror will hit the lens. I'm fairly certain that generally speaking, it's better to put the lens as close to the sensor as possible, so this is probably an overall good thing, though it does cause the problem of not being able to make use of the EF-S lenses on full frame bodies when shooting video. There's always a compromise.
Comments, corrections, other ways Canon's mount effects photography, and so on are welcomed!