How does the severity of barotrauma vary with rate of descent?
It's purely subjective, but I can tell you from personal experience descending in unpressurized airplanes that a 500 fpm descent is WAY less painful than a 1500 fpm descent if you have something like a blocked sinus passage.  The blockage is usually not 100%, so the pressure rate of change has a huge effect on the pressure differential that is allowed to build up in your head as pressure equalizes, but just not fast enough.

Your head and sinus cavities (or inner ear) are effectively turned into a human Vertical Speed Indicator (which simply measures the rate of air pressure equalization between an aneroid chamber and ambient static through a restrictor, like your blocked sinus), but instead of a VSI pointer on the dial, you have the pain receptors in your skull.  Like the VSI, a 1500 fpm descent will crank your pain receptor dial quite a bit higher than a 500 fpm one will if you have a sinus infection.

The important thing to point out however, is the typical airline passenger never experiences anything more than 500fpm in normal operations, because pressurization control schedules limit the rate of change in cabin pressure to that value.  In any jet you are normally descending anywhere from 1-3000 fpm on an arrival, and even on final on a normal glideslope at, say 130kt, you are descending somewhere around 750 fpm.  

It's only in unpressurized airplanes do you generally try to limit climbs and descents to 500 fpm when passengers are on board.