Biggest difference between XTR and XT is that the XTR brakes pivot on ball bearings while XT uses the traditional brass bushing. The bearings are really nice since there's no play in the brake arm whatsoever. Same thing with Dura-Ace vs. Ultegra ... the Dura-Ace calipers have ball bearing pivots while the Ultegra and lower have bushings.

If you look closely at the XTR brake arm, you'll see a 6-sided nut just below the brake arm mounting bolt. That's the bearing adjustment nut.