Larisa wrote an interesting post on difficulty in WoW entitled Why I Don't Want to Hear Another "WoW is Too Easy" Statement. Her response sums up a lot of my feelings on this issue.
However, I've been thinking a bit more about this issue, and have been looking at guild rankings at Guild Progress. I've also been reading a lot of Regency novels lately, and am inspired to make an analogy which may or may not reflect reality.
In Wrath, I think PvE guilds can be categorized as follows (for raiding purposes). All numbers are approximate:
Royalty - The two hundred guilds which can clear everything, including Trial of the Grand Crusader.
The Aristocracy - The three thousand guilds which can defeat at least one boss in TotGC--or some Ulduar Keeper hard modes--and thus are working their way through hard modes.
The Gentry - The ten thousand guilds which can defeat regular Trial of the Crusader, but haven't been able to advance into the hard modes.
The Bourgeoisie - The next ten thousand guilds which are working their way through Trial of the Crusader. Also includes those guilds working on Naxxramas and Ulduar. Basically any guild that is still working on normal difficulty content.
The Proletariat - Our beloved casuals. All the other guilds which are levelling or making alts or doing 5-mans, and haven't really gotten into raiding yet.
At different points in WoW history, different sections complain about different things. In Vanilla and TBC, a lot of the complaints about raiding came from the Bourgeoisie, who were unable to break into raiding at all.
However, I think the current complaints about WoW being too easy are coming from the Gentry. They can beat regular modes fairly easily, but hard modes are completely beyond their touch. This means that they see hard modes as something for the "crazy hardcore" and not for "normal people". And the problem is compounded in that a lot of people who are active in the WoW community via blogs or forums come from the Gentry.
Whereas both Larisa and I are in guilds which are in the Aristocracy, which can beat some of the hard modes. So to us, complaining that WoW is too easy smacks of whining. The content is there to be beaten.
The real problem is that the Gentry is currently too large. Too many guilds are in that gap between hard modes and regular modes. It needs to be shrunk from both ends. The difficulty of the first two bosses in TotGC should be reduced a little bit, and the difficulty of the last two bosses in TotC should be increased a little bit. That should create a more gradual path. Ulduar is a fairly good example of this, as the jump from Yogg-Saron to the first few hard modes is not that large.1
The Gentry need something to work on, otherwise they will be unhappy. Ideally, a lot of them should be in the lower ranks of the Aristocracy or upper ranks of the Bourgeoisie. Too many guilds in that gap causes a lot of complaining.
1Actually, I think Yogg+4 could be reduced in difficulty. It seems just slightly too hard for it's placement. But it's not all that bad to have a hard end boss.