The answer of "Polishing material choices and machining processes .." pretty much sums up the differences. Higher line hubs will be lighter, have a better polish and exterior finish and have better quality races and cones (or bearing cartridges for that type).
Within the Shimano road hub family the differences between 105, Ultegra and Dura Ace are not dramatic. 105 hubs can give excellent service and be very durable but they will be heavier and not have as nice an exterior finish. DA hubs are beautifully finished inside and out and have a lighter Ti freehub body in place of the steel body on the lower lines. The MTB hubs LX, XT and XTR follow pretty much the same weight and finish progression and, as noted, are better sealed than road hubs.
Are DA and XTR worth their significant cost difference? Your call.
fietsbob does make good points with his questions. What are you going to use the bike for? Are you a casual rider who only rides occasionally or a committed road junkie who puts 10,000 miles a year on his bike(s)? Is your budget tight or lavish? How much does "bling" mean to you?
Short answer: 105 or LX will give great service. DA and XTR will make you proud to own them and also give great service,