Nikon is out simply because figuring out which lenses and which accessories work with which bodies seemed to confusing for me. I tend to stick with that which is nearly self explanitory. Which is why I bought a Maxxum as my first SLR - Maxxum lenses for Maxxum cameras. It's cut and dry - I can't goof and get something that won't work.
Even the Pentax K-mounts are confusing for me, but I've found it's not so bad that I can't live with it.