Adjustment opportunities, power, modulation, single finger lever design, reasonable price and build quality.
Not as fantastic as some reviews would have you believe, expensive gimmicky finned pads, need for Shimano's own hydraulic mineral oil, worry about brakes not heating up in cold weather, Shimano's (aftermarket) own rotors can be a bit sub-par .
Having used quite a few brakes I was drawn to the XT brakes due to a reasonable price and stellar reviews. Having used them for months I have to agree they are certainly a well made brake that is powerful and modulates very well. Props to Shimano for making such a good brake, but more than that such a great range of brakes. Honestly though I have a M596 paired up on the rear and another on the front of another bike and it really is 98% as good as the M785 (minus a bit of power and a few adjustments), perfectly adequate for most XC and trail riders. Also many reviews say that the XT is super powerful, but I find that unlikely compared to a four piston Saint brake for example. Also I have some worry that with all the ice-tec and finns in our -10 winters the brakes might take a bit long to warm up.