Speaking for me here, the reason I bought Nikon instead of Canon is that Canon has changed it's SLR mount several times over the years, rendering previous lenses unusable on current cameras. On the other hand, the first Nikon SLR lenses from decades ago still work on many current models. When that first happened, I didn't have either brand, but still noted the flexibility of Nikon's attitude vs Canon's implementation of a polivcy modern industrial obsolesence, a concept which no one in their right mind should respect. Being of right mind, when the choice came, I chose Nikon, and have not been disappointed. Almost every lens I own currently was made for the older Nikons, pre digital. Try that with Canon.
That all Nikon lenses can be used on Canons, but not all Canon lenses? That's supposed to be a strong poing for Canon??? To me it looks like they can't make their own system work. That's not cool at all.
Another very small point for those concerned about aesthetics: Nikons look like cameras. Canons look like Barney the dinosaur with a lens stuck in his nose: shapeless blobs. So it appears they not only don't have competent mechanics on staff, they don't have designers, either. :-)