In a forum post earlier today Blizzard WoTLK Gold stated that they are now "significantly cutting down on the speed in which health for creatures increases in line with the level of your item." In actuality, this feature is already available in World of Warcraft .
"All we're trying this is stop instances that monsters die too fast for players to react to their presence, particularly during World Quests," Blizzard explained. "Our initial tune was much more aggressive than what it was supposed to be. This may create the impression that your upgrades weren't actually improving your game. This modification will make it quite clear that you're nevertheless becoming more powerful than your enemies as your gear increases."
Another noteworthy change currently in place is that Blizzard has cut back value of damage dealt by enemies. "Our main goal with this change is to prevent the the time required the creature to die from becoming ridiculously short, and increasing the amount of damage they cause is not necessary for this goal," Blizzard said. "Instead this felt like they were becoming more dangerous. This was not our intent."
Blizzard continued to state that some of the mentioned modifications were available in World of Warcraft through the 7.2 Public Test Realm patch, and even date back to January. The developer hasn't informed buy WOW WoTLK Classic Gold players about it right away since the studio needed "raw accurate and genuine feedback."