While Karazhan was one of the (if not the) most successful instances ever, it had one big problem: it killed guilds. It murdered progression. It was a roadblock after a roadblock, so much so that it took some guilds months to conquer, if they survived at all. Ten man Naxx obviously doesn't have that problem -- anyone with a little raiding experience who wants to beat bosses in there can do so, and Obsidian Sanctum is just as easy. The problem now, however, is that guilds like Ensidia and guilds who pushed through to Sunwell in the old endgame, are finishing the content already, and wondering what's next? They were 80 two weeks ago, and now, barely a month after the expansion's release, they've toppled every dungeon they can find.
And what's wrong with that? Nothing.