That's funny stuff considering how it seems like they lowered the game's difficulty settings with each patch. For instance, I remember how difficult the plague lands instances were in beta about at the end of the first year of the game being live they got significantly easier (and from what I read they are even easier today).
I think their point with what you quoted is that there's been a significant rise in third-party game resources as the expansions have progressed. You have a lot more easy-to-find resources for gearing, rotations, and boss encounters now than you did back in Vanilla or TBC. You have much more information on how to do things efficiently.
I strongly suspect that another part of the problem is that they spent so much of their resources updating the Old World instead of making more new content. And I'm not entirely sure I wouldn't've made the same decision; people would often drop the regular quests once they hit max level and only do the dailies they could do and dungeons, so a good chunk of TBC and Wrath content that people could have done was just sitting there because they would then go and run their daily heroics (getting faction rep via tabards), do the appropriate dailies, and go on about their day. And the questing in the Old World was significantly more painful than it was in TBC or Wrath. So, on a cost-benefit analysis
The problem with only having 5 zones for Cata, though, is that the questing is now very, very linear. So you're going to repeat the same series of quests on each and every alt, because there aren't any other options. In Wrath, you had several zone paths to mix and match, as you did in Vanilla. Now? It's a straight line. Your only real choice is Vashj'ir or Mount Hyjal to start with, and if you're a primary caster, it's most efficient to do Mount Hyjal. This also creates the perception that there's little to do. Once you're done with Twilight Highlands, that's it. You've got whatever dailies you can find, and your dungeons.
That is, in fact, yet another problem. After Wrath, everyone was used to the Dungeon Finder, which was used very extensively during the second year. So, instead of waiting in a city to find someone for whatever the daily heroic was, you could queue up for a dungeon and do your dailies or undone quests or farm or all of the above. And, by that point, a rather large chunk of the population was significantly overgearing the existing heroics, so you could blow through the daily in 15-25 minutes, with no crowd control or even any sort of planning. Which got missed, and on the forums you'd see people keening and crying for crowd control requirements.
This is because people don't remember things. At the beginning of Wrath, you could generally use some CC. Not always, but you generally benefited from having a sheep or sap or something out, especially with a new tank or healer. After everyone outgeared the instances, though, it became more efficient just to AOE stuff down. And, so, the perception became that Wrath dungeons were always AOEfests.
So, Cata launches. Crowd control is necessary, so coordination is necessary, which is much harder/takes longer if you don't have some form of third-party voice chat. Combat ratings (i.e. haste, crit, hit, etc) took a nosedive as you leveled up, so damage as a new 85 was quite often lower than it was when you were 80 in your full ICC-25 gear. So, you have a bunch of people who're barely geared for the content going in as soon as they can, and despite the developers being very clear about these being significantly harder than what they're used to from Wrath, they wind up getting curbstomped by bosses that, based on their history in Wrath, they feel they should be able to roll over.
Once people were able to gear up sufficiently, and learn the encounters, they became much easier. Some encounters were, in fact, overtuned, and were adjusted. BUT, I am convinced that a lot of people who were, in fact, not really prepared, wound up giving up on the game because it's not what they were used to (this is mostly anecdotal from work friends and such...but I also notice a lot of my guildies who weren't really that good took long breaks).
Obviously, that's not true of everyone. The game has been out a long time, and it's quite understandable that people will get tired of it. But I'm positive that the dungeon and questing changes had a lot to do with it. Hell, I was about to quit until I was able to secure a regular spot on my guild's raiding team.
Brad