Part of it is the method used to keep the lifter in its proper position. The late model OEM hydro lifters have two sides of the lifter machined flat so that the lifter looks kind of like this from the top:
A flat piece of steel with two silimarly shaped holes fits over each pair of lifters. A piece of spring steel bolts to the center of the lifgter valley and holds each lifter retainer in place. Because the lifter is not round in the area around the retainer, it can't turn in the lifter bore.
Several people have had problems with the rivets that are used to attach the bars to the lifter body on the aftermarket lifters.
I recently read an article in a mag about how the guys running in the crate motor classes use the stock hydro roller lifters. They do a modification that takes a lot of the hydraulic movement out of the lifter - almost, but not quite, making them solid rollers. This allows them to rev the engines over 7,000 RPMs without problems.