If memory serves there is only defensive benefits.
Warriors, Death Knights, Demon Hunters and to a lesser extent Druids, Rogues, Hunters, Warlocks, Priest, Paladins and Mages all get some form healing/absorbs that scale off of HP with the first three I mentionned benefitting from it the most as it is their primary way of healing on already tanky classes.
Other than that Prot Warriors do get an increase to armor but I believe it is based off of strength so no help there and technically pet healing abilities do scale with HP but your pet HP already does do thay so it's not that impactful either.
Offensively I don't remember any ability other than Touch of Death.