Canon and Nikon really are a horse a piece. Each has their strengths and weaknesses, but they both have good SYSTEMS as a whole.
Sony could have the greatest sensors in the world, but they don't have lenses, they don't have flashes, and they don't have third party support. In the current market, I could never recommend that someone buy a Sony dslr, no matter how good the sensor tests in DXO.
OK, just browsed their more-fancy-than-functional website, there is more depth to their lineup than last time I looked; so I'll gladly eat crow on that one...
...still wouldn't buy one though.
Commitment to the market is a good point. Sony has a long history of coming up with proprietary versions of standard technologies (DAT, Mini-disk, ATRAC, Memory Stick, HiFD, UMD, SACD, BETA, etc.); and dumping them when they don't take off as well as hoped. They also have a recent incident of being hostile towards customers (rootkit fiasco).