At the start of cataclysm alot of people hit 85 very fast, within a couple days, I know I hit 85 within a week. I was very shocked between the heroic difficulty of wrath and cataclysm. Trash pulls made almost every healer go oom waiting for them to drink. Having to CC was something new to alot of people. I think the dungeon I had the least success in most pugs was grim batol because of basically every boss had some mechanic besides forgemaster that could generally just one shot someone from 100% to 0%.
In Mists of pandaria this is not the case, not even in the slightest. Tanks are soloing heroics basically, dps/healers run the whole dungeon without a tank. Given even in tier 12 gear (nobody ran regular cata heroics unless they couldnt run troll/hour of twilight come 4.3) you still needed a healer & usually a dps or 2 to down most of the bosses in most 5 man heroics. Alot of people have horrible memories in the 4.1 troll heroics. Then 4.3 came and the heroics were just faceroll. Sure you had a mechanic or 2 to abide by, but if you made a mistake you weren't highly punished for it. This is pretty much the case in MoP.
Whats your preference in the two? Mine is cataclysm, yeah its less pug friendly, but when I killed erudax for the first time (I was in a pug) I felt awesome. I remember the first heroic dungeon my paladin (my second 85 only within 2 1/2 weeks of cata release) entered was deadmines. That one was a rough one for alot of people as well. The first thing I said was "Well we'll see how this goes, I haven't had much luck with this on my dk yet." No matter, we cleared it, I was exstatic.
When MoP came I wasn't very impressed with the difficulty. I RaF granted my monk to instant 80 then leveled straight from there to 90 on MoP launch. When I hit 90 I was MW main, and let me tell you...damage was extremely low. I was confused, was I in a good group? Maybe this heroic is just easy. So I queued up for a few more with randoms. Same thing, I was really confused. I knew they wanted to make them more casual friendly but to this level?
Regardless, lets hear your opinions. Alot of people have mixed feelings about this and its been on my mind the past couple weeks.
Note: I did not include wrath heroic difficulty level as its relatively close to MoP, even at launch, and I don't personally know alot of people who did BC heroics nor were they playing at the time of current 70. That and I felt they were fairly close to cataclysm launch 5m heroic difficulty.
Additional Note: This is launch specific speaking. Within the first day to the first 6 months or so. As I said after tier 12, people basically just facerolled cata heroics if they had a good tank/healer/dps.