Any MMO needs macroeconomic balancing to prevent runaway inflation. I like the LOTRO system where expensive mounts, crafting, and player housing pull money out of the economy, keeping prices fairly balanced.

I think games with PvP or RvR action have great potential to pull money out with purchasable fortifications/ defensive buffs, etc. EVE handles this issue by allowing PVP to destroy very expensive ships.

I prefer the route of economic balancing that relies on voluntary purchases such as housing, mounts, or PVP ships rather than an enforced cap on population or fixed amounts of currency. This style works well in LOTRO and EVE, at least.