Tremallyn of Shu'halo is actually asking for scalable instances -- he says that if 10 and 25man groups are going to be playing the same instances in Northrend, why not scale those down to five man and even solo versions, so all players can get a chance to see the content? Bornakk replies that the rest of the game already is solo content, and that they want the endgame dungeons to be more exclusive. We've heard other good reasons before -- to tune an instance for every class, with all of their varied strengths and weaknesses, would require Blizzard to water down the instance so much that it wouldn't be nearly as dynamic an experience as you can have with a minimum 10 people in the party.
Not that it's impossible -- other games (I know of one in particular) have scalable instances, some of which are even tuned for one player. But at the same time, we play MMO games for a reason -- if you really just wanted to play a great single player experience, there are a wealth of single player games to choose from out there. And if you really want to do an instance solo, you could always just wait 10 levels and do it solo anyway.