I was thinking about this recently, given that I'm looking forward to the Classic development. Classic, and mainly The Burning Crusade, didn't have multiple difficulties in order to capture as many people as possible, irrespective of their achievement intent or general skill/ability. Classic, for example, had a series of five-man dungeons of a single difficulty, and multiple raids (which differed only in person size).
This is how we now look:
Leveling 5-man dungeons.
Level cap 5-man dungeons.
Heroic 5-man dungeons.
Mythic 5-man dungeons.
Mythic+ 5-man dungeons.
As far as raids are concerned, we're here:
Looking For Raid multiple-man difficulty.
Normal multiple-man difficulty.
Heroic multiple-man difficulty.
Mythic 20-man difficulty.
For design reasons, it's easy to work out why an additional seven difficulties for identical content has been put in; but that's not all. The achievement system is now ludicrously bloated, the profession skills have been gutted, quests have been turned into single-player easy completions and gearing has been expanded to a fundamentally broken extent. In order to expand this point, gear items that you pick up from Mythic dungeons are doubled in strength from those picked up from level cap, and it's a similar deal in the raid system. Hell, due to random enhancement, it could technically be more.
So the question is simple, really.
Do you think this is right?
Do you think there should be heaps of difficulties in order to 'encourage' more people to do certain content while others are degenerated, or should this be properly reviewed to make content completion an achievement, rather than entitlement?