Minecraft isn't WoW (that's a good thing). WoW is pretty much aggressively targeted to make large amounts of money, which is why they're valued so high. Mojang seem to want to make things that are cool and fun, more than they want to set up large subscription bases, add on packs and so on. Could Mojang become a billion dollar company? Sure, maybe. Anything can happen. Is it a good comparison between them and Blizzard? I'd say no, they seem to have distinctly different goals.

You know what's cool, being a company that is happy making things rather than being a billion dollar company and dealing with the headaches of stock price related soothsaying.