This whole list was brought up by the old argument of whether or not WoW is too easy, something that seems to come up periodically, especially whenever a new tier of content is released. Actually, we're talking about whether raiding is too easy here, and BoK says that there are too many people in the "Gentry" category he defines -- too many people who've finished off normal modes, and who seem to think that hard modes just aren't for them. I guess the solution there, besides, you know, more content, is that instead of making a huge difficultly ramp from normal to hard modes, Blizzard should tweak up the difficulty of the last few bosses in normal, giving "the Gentry" a chance to get a taste of tougher content before they move up into the hard mode tier.
The good news is that at this point, we're really talking about variations on a boss level rather than an entire dungeon -- back when Naxx was originally dropped into the game, the problem was that no one could run any of it unless they were in the upper level "Aristocracy," and Blizzard has certainly fixed that problem. I think in the future that we will see even more levels of difficulty. I'm not convinced that Blizzard will stick with the 10/25/normal/Heroic model (in fact, we've already heard that the toggle will mix things up in Icecrown), but I'm sure they'll continue to give us more and more ways to run content the way we want, rather than moving through a required order or gateway.