If you've been playing (and trolling the forums) for any amount of time, you'll know that the argument made here -- for challenging, instanced, single-player content -- is not a new one. This thread has all the usual responses, ranging from definitions of "massively multiplayer" to suggestions that solo instances would be impossible to create to suggestions that class-specific solo instances would be perfect. However, there's a lot of good discussion in the thread -- highlighted by a post by Tseric attempting to explain why this type of content is not feasible.
If it requires too much healing to complete, pure melee or casters are impeded.
If it requires significant amounts of dps, you either have to spec and gear properly for one, single dungeon or pure dps classes simply have a leg up.
If you can't tank or mitigate damage effectively...well, you're dead at the first pull.
Therefore, if you reduce group size and composition to a single participant, the more generic and easy you have to make the dungeon.
And of balancing nine class specific dungeons? Tseric quips, "Do you realize how much time that would take?" And you have to admit -- he's got a point. Would you rather see Blizzard spending their development time working on nine different variations of the same dungeon, or working on nine new dungeons everyone can see?
Of course, as painful as my last PuG was, I might prefer the solo content anyway.