I believe the higher end hubs have mostly gone to an oversized alloy axle, and the bearings now have a larger number of smaller balls in them in order to make room (I don't know why they didn't just make the hub body and flanges slightly bigger -- maybe weight). The axle is supposed to be stronger and less prone to bending on impacts. But some posts I've read claim that having more, smaller balls increases rolling resistance slightly and possibly reduces durability under heavy loads. The lower end hubs still have the old, proven, steel threaded axle with 1/4" balls, though even then you will read claims that the lower end hubs have lower quality cups and cones, which might negate any advantage.

So if these hypotheses are true, Shimano gave with one hand (stronger axle) and took with the other (weaker bearings, more drag). I don't know which is more important. Probably a toss-up.