Tuesday is the big release day for the Burning Crusade expansion for World of Warcraft. It's the first real expansion for the game since its release more than two years ago. And there are 8 million people playing WoW. It's a big deal.

The expansion is a huge game balance gamble. Traditional RPGs have a problem: you only have so many employees and so much content. So after the players have levelled up and advanced and played all your content, what do they do until you can ship an expansion?

The usual solution is to slow the players down, make them replay the content you have. Warcraft does this admirably with a huge amount of gameplay at level 60. Hard dungeons with finicky boss fights that take months to learn, 50+ hour reputation grinds, PvP minigames, etc etc. All gameplay that gets players to happily work for weeks for powerful rewards. The level 60 game works so well that many serious players consider levelling from 1-60 as a mere prelude to the real game.

But Burning Crusade upsets this careful balance. Now 60 isn't the end, there's new content and levelling from level 60 to 70. Which is great, but that progression is only going to take people a few weeks. And then what? It's not really clear that the old level 60 gameplay is going to scale well to level 70 players (although they are trying). Presumably there will be a new grind at level 70, but will players go for it? Particularly after the bitterness of seing their hard-earned level 60 loot so quickly obsoleted?

It's a big gamble, but I bet it will work. Blizzard are masters at scaling the rewards for players, and I suspect the new grind at 70 will probably just seem like new fun for everyone. We'll know in three months if it works.