Originally Posted by andymac

Downhill rims are generally really wide to accomodate tires up to 3 inches wide and the rim often has a heavier build as being able to pedal a downhill specific bike is not really that important. The plus to running a really wide rim is that when you are running "normal" tire widths (i.e. 2 inch slicks) the sidewall of the tire is not bulged out, I have found this helps with preventing sidewall damage on my tires.

A downhill oriented rear hub is usually built a bit heavier as well, often with a bolt on axle rather than quick release and the pricier units have more engagement points in the freehub so they are in theory less likely to slip. The front hubs use a 20mm through axle for added strength/stiffness.

For commuting these sorts of wheels are usually overkill and the additional rotational weight is not appreciated when trying to get up to speed.