TV is essentially a passive experience. You watch what someone else has produced. And while it can be a communal experience, you normally have to be in the same room as the people you're watching TV with. In WoW, the same restrictions do not apply. You're interacting with the content. It's designed so that you have to do stuff to get the most enjoyment out of it. You are the protagonist. And, more than that, you're online with thousands of other people at the same time, often working together to achieve the objectives within the game. And I've managed to stack things such that most of the people I play with are my real life friends. Indeed 90% of the guys on the stag night last weekend were also my guildmates in WoW, including the groom. (The bride's a player, too…)
That combination of the social and the interactive is what makes the game very compelling – and all but the best TV rather insipid by comparison.
Don't believe me? OK: ask Ozzy: