Take all that work you put into your characters, the gear, the raiding, etc. for MoP your joy for the increased numbers once you got a new piece of armor or better yet, a better weapon. Take all that and nerf it to the ground, to Wotlk level, one day you were doing MoP numbers, the next day you are doing the same dps as some alt you don't care about that's stuck in Northrend. Hell, your hp, dps, spell numbers will be smaller than of those of your current Cataclysm alts.
They can BS all they want that it will be percentages etc. but at the end of the day, it's a nerf.
Numbers indicate progress, take that away and you remove the incentive to progress when you know any new gear will pull off far smaller numbers than MoP or even Cataclysm. This move by Blizz will cost them many many subscribers, so it won't take long until they remove the squish. Ofc a vocal minority will scream outrage, but Blizz should stop letting their spam on the forums determine the direction of the game!