As franky1 has pointed out, there are certain rules that the miners have NO CONTROL over. For those rule, it doesn't matter how much computing power they can contro. They could have 99.99% of all the block hashing power in the world, and if they tried to break any of those rules, ALL of the remaining 0.01% of the mining power AND the non-mining nodes, merchants, and users would simply ignore every block that they created. They'd have spent huge amounts of money to aquire their equipment, huge amounts of money on electricity to run the equipment, huge amounts of money on locations to store the equipment, and huge amounts of money to cool the equipment, and in return they'd get nothing.

However, this doesn't mean there's nothing they could do with that computing power to have an effect on the Bitcoin system.  It just means that there are certain things that they can't do (such as increase the amount of new bitcoin that gets created in each block or include transactions with invalid signatures in their block).  Some malicious things that they COULD do if they control more Bitcoin hashing power than the rest of the world combined (at least until a significant number of Bitcoin participants decided to create a new system that excluded them) would be to choose to block certain transactions from ever being confirmed, or to mine 100% of all the blocks (keeping all the block rewards for themselves and making it impossible for anyone else to mine any bitcoins) or to choose which other miners or pools they want to allow to participate.  They'd also be able to completely erase recent blocks from the blockchain and replace them with other valid blocks that they prefer.