If you're not much of a computer programming person, this one might make your eyes glaze over a bit, but if you have any interest how the AI of videogame characters, including those in WoW, is programmed, this article about designing AI pathfinding is a terrific read. "Pathfinding" is a method of determining how NPCs move within a game world like Azeroth -- you and I can clearly see where the walls and bad guys are, and so we just have to press buttons to avoid either ingame, but NPCs (including pets and mobs) aren't quite that easy -- they need to be told clearly by programmers where they can go and how to get there. And when the rules they're given don't quite work, you get the funny seen above.
Many games use a "waypoint" system -- NPCs are given a series of paths around the space they can move in, and use those paths to determine where they can and can't go. The article argues for a "navigation mesh," a much looser definition of available space, which NPCs can then draw their own path across. It's a little technical, but it's cool to see the inner workings (and weaknesses) of Azeroth's code.
Of course, it's extremely unlikely that we'll ever really see the NPC pathfinding engine updated in WoW anyway -- Blizzard will update their system in certain places to fix things like exploits (and the occasional annoying escort quest, i.e. all of them), but there's no real need to update the whole system completely when there's so much content to be done. Hopefully videos like this will bring the problem to light, and in future games we'll see some better pathfinding. Someday, that NPC will know that it's easier to go around the pillar rather than trying to walk right through it.