I like the AOE cap and hope it stays. I guess players who did massive amounts of AOE damage might feel cheated now that they might have to actually compete with others not endowed by Blizzard with ungodly uncapped AOE damage ... but that is a good thing in my book. I was doing a M+20 last night with a hunter and DH and something amazing happened... our overall DPS was within a percent of one another. Prior to this change the hunter and DH would've been way, way, way above me in overall DPS... so the AOE cap is a great balance change... Also, I actually agree with Blizzard... pulling huge packs of trash and burst AOE'ing them down kind of sucks.
As for farming old content with the AOE cap... I honestly didn't see a difference at all in the amount of time it took. It probably was slower, but it was such a small change in the pace that I didn't even notice. That being said, if they decided to remove the AOE cap for older content I wouldn't complain... it's not like that stuff matters anyway, I doubt anyone decided who to main by how fast they can clear a past expansions raid/dungeon content...