Despite the good responses, there are two points that I believe are important that haven't been pointed out yet. 1. While a 51% pool operator could double spen

Despite the good responses, there are two points that I believe are important that haven't been pointed out yet. 1. While a 51% pool operator could double spend, none of the participants can do that. The social incentives against a pool operator exploiting this power are very strong, however, because double spends are basically impossible to hide and would destroy the value Bitcoin - which is pretty much against the operator of a mining pool who is likely to hold significant amounts of Bitcoin herself. 2. There is a second possible exploit that is not so well known, which is that a >50% pool can capture all mining rewards. They can do so by never accepting a block mined by somebody else, and instead always mining on their private chain. This pool-specific chain will always ultimately become longer. The loss of confidence associated with such an attack is probably smaller. So I would bet that if the pool operator has evil intentions, that would be the way to go for them.