A few points that haven't been mentioned about Sony's cameras:
- The only way to get autofocus Zeiss lenses1. While Zeiss makes lenses for Canon and Nikon mounts, they're strictly manual focus.
- Yes, old Minolta lenses work -- and many are almost amazingly good on digital. Just for one example, there was a recent comparative review of the old (circa 1985) Minolta 70-210/4 to the current Canon 70-200/4L -- in which (at least according to that reviewer) the old Minolta did quite well. I feel obliged to add that the reviewer in question is generally a Minolta advocate so there's probably some bias -- but the pictures he includes do seem to support his conclusions at least reasonably well.
- Focus Peaking. The recent SLT models have a focus-peaking mode, where the EVF and/or LCD will highlight what parts of the picture are in focus.
- Although some people obviously disagree, as far as I can tell, Nikon's are really only designed for people who are ambidextrous, and Canon's seem to have been designed for some alien race with hands that have each have 2 (or maybe three) thumbs and 6 fingers, all with lengths quite a bit different from any human's (i.e., Nikon has a habit of requiring you to do things with both hands at once to accomplish things, while Canon seems (to me) to just scatter buttons almost randomly, so things get changed by accident constantly, but are hard to reach and/or manipulate when you need them).
- Higher color saturation and accuracy. Nikon and (especially) Canon use "thinner" (less saturated) color filters on their sensors. This reduces noise but also reduces the range of colors they can sense.
- Fast (phase-detect) AF in video mode on recent SLT models (A77, A65, and probably A57).
- Liveview that isn't a clunky mess.
- Mirror lockup implemented for actual use instead of (apparently) to just fill in a spot on the feature list (e.g., most Canons that have mirror lockup at all bury it under a custom function menu where it's hard to find and harder to use).
At least to me, one of the most important features is really the lens selection. Unlike most brands, where lens selection is almost entirely about sheer numbers, with Sony you get a choice of character.
Back when German companies produced all the best cameras/lenses, Zeiss produced the sharpest, highest contrast, most technically advanced lenses you could get. Leica's lenses had lower contrast but high micro-contrast, designed more toward taking "prettier" pictures.
At one time, Minolta was closely associated with Leica2 and followed a a similar lens design philosophy. Sony, on the other hand, has used Zeiss lenses on most of their pro video cameras for quite a while, and now has Ziess lenses for their still cameras as well.
As such, lens selection for Sony cameras isn't just about buying the best lenses you can get, but about selecting lenses to express your vision for a particular photograph (or type of photograph). When you want the almost-glowing look of a Leica, you can get that with the older Minolta lenses. When you want the sharpest, most detailed, highest contrast, etc., pictures, the Sony Zeiss lenses will be a better fit. I suppose I should add, however, that buying both could get just a bit expensive. Most people consider one 85/1.4 (for example) a pretty expensive lens; few are likely to buy both a Minolta 85/1.4G and a Sony/Zeiss 85/1.4 ZA (though if I could afford it, I probably would have and use both).
If you want to get really technical, there were also a few Contax/Yashica AF Zeiss lenses, but they've all been obsolete for quite a while. Excellent lenses, but your body choices are film or one digital body, which may be the best 6 MP camera ever built, but it's hardly competitive with anything recent.
Closely enough that, for example, Leica CL and CLE cameras were actually made by Minolta.