One of the things that comes with most high end DSLRs these days, but may be missing on the mid level ones is 14-bit capture. Some of the Nikons allow switching between 12-bit and 14-bit (sorry Sony, Olympus, and Pentax owners - I don't know enough about them). I've seen a lot of comments in reviews that it theoretically allows finer gradation in the shadows, etc. but had failed to produce an example of the difference it actually makes. Most people who have compared say they can't see a difference.
A comparison between the two on Earthbound Light
does find a difference, but it also seems to explain why it won't make much difference for the majority of photos. My camera (Canon 40D) does have 14-bit capture and I haven't noticed a difference in shadow detail over the one I replaced (Canon 20D) - but given that I can't go back to 12-bit and save file space it is kind of a moot point for me. My take after reading on this subject is that it doesn't make much difference - unless you are seriously pushing exposure multiple stops. I suppose if you do a lot of low light shooting that may be an advantage, but I still think it is a very minor point when deciding on a camera. The bits are wasted on highlight detail (already enough) and noticeable in shadows only when pushed brighter.
Am I happy it is on my camera? Sure - drive space is cheap and it still shoots at 6fps so I can't really complain about speed. Have I noticed a difference in practical terms? No. Would I purchase a camera that 'only' had 12-bits for stock? Of course! (Keep in mind that JPEGs only have 8-bits in the end and there are a lot of very successful photographers that never touch a RAW file.)
I'd be interested to know how many people with the higher end Nikons leave their cameras in 12-bit instead of 14-bit...