What happens if a monopoly grows in Bitcoin mining and payment processing?
In that scenario, the corporation could:


Launch a 51% attack at a whim
Pretty much control the Bitcoin economy, driving the prices up or down how it sees fit (at least until it spends all of their Bitcoins)
Control which transactions get included in the block or not. This could require one to use their specific client and thus gain more control over the network
Have a big impact on the Difficulty, as it could turn its machines on and off
Control the rate at Blocks are mined (for example, holding onto a solved block for certain amount of time before broadcasting it)
Control who gets to see the most recent Blocks. With this, they could hold out a long chain of solved Blocks for their own client, and when a standard client would finally mine a block, they could just release a couple old blocks to invalidate that chain


All in all, if you own majority of the hashing power, you can pretty much dictate your own rules, up to what the Protocol allows.