I don’t know about the revamped low level experience, but the 80-85 leveling isn’t exactly easy. They made the new mobs hit HARD, and using interrupts is now key to your survival. As a DK, at least, I used to be able to pull 4-5 mobs my level or above and hold my own relatively easily. It was hectic, but not very difficult. That does not fly anymore, even taking mobs a level or two below me is difficult if you get to about 3 at a time. Chaining mobs isn’t as hard, but I can’t do it anywhere near endlessly anymore, there’s an upper limit at about 5 or 6, or lower if it’s a particularly hard hitting mob (often a caster), to the point where making my ghoul offtank sometimes actually happened (I never even considered this option before Cat). And I had trouble with quite a few quests in Twilight Highlands at 85, these were early in the zone, so it’s easily possible to experience this while leveling if you skip a zone or leave it prematurely. It took some sitting back and observing the mechanics of the area to understand how to make it easy enough to be manageable, and even then it’s possible to get fubar’d if you’re unlucky enough. (For those in the know, I’m talking about the Thundermar Ruins quests, horde side at least).
I have a feeling that they may have just changed the learning curve at bit, lower level mobs are more trivial and higher level mobs are angrier. This is generally in line with Blizzard’s philosophy anyway, pre-Wrath they nerfed the hell out of everything in Outland, making quests that would previously devastate my characters incredibly easy, with a couple exceptions. Not that Wrath content was difficult by any means, but it was certainly average, compared to the nerfed Old World and Outland content. Even if it’s not truly harder, it requires more… attention now in my opinion. It’s probably around the same difficulty if you understand how to use interrupts or avoid pulling extra mobs, but that’s more than we can say for pre-Cat content.