Probably the RNG aspects of the cache. Personally I think if the rewards were less RNG and more guaranteed then paragon rep would of been more fondly remembered.
My idea for paragon rep in the past was to do it in a tier level fashion. Each level of paragon rep would have rewards to it like for example tier 1 would give an heirloom armor token allowing it to be boosted to 120, tier 2 a weapon heirloom boosting token, tier 3, a rep faction specific item, tier 4 a mount and tier 5 a repeatable to infinity gold tier or something like that. Obviously the items can be improved upon but the basics would boil down to the following points.
*Guaranteed non rng rewards
*Something OPTIONAL (not player power) to work towards so no one feels "forced"
*Something that is endless without feeling punishing.