If someone had the majority of the mining power..

[skooter]

Like let's say someone has 75% of the overall mining power.

Could they reject other miners' valid blocks? The 75%'s blockchain will always be longer then anyone else's, so would it cause all other miners' blocks to be orphaned?

And they could essentially set rules on the transactions, and set a minimum Tx fee to get a Tx included in the blockchain.

[jockular]

this is a well known issue, and you only need 51%, likely less

[Beliathon]

We'll know where they live, and we'll be able to stop them before they ruin the blockchain. By force, if necessary.

But it won't be. Because once fiat money has died, there will be so much hashing power thrown at Bitcoin that getting to 10% will be impossible, let alone 50%.

[skooter]

Well, in that case it's pretty easy for government(s) to set rules on transactions.

Most of the big mining facilities are in developed 1st world countries, so it'd be pretty easy to force them to comply, or if they don't the govt can seize their hardware.

[Equate]

All the blocks you mine are orphaned and above 51% attack will fork the coin.

[DannyHamilton]

Like let's say someone has 75% of the overall mining power.

That would be an awful lot of hashing power.  It would be very difficult and expensive to acquire.

Could they reject other miners' valid blocks?

Yes.  As a matter of fact, if they wanted to do so, they could manage it with anything more than 50%.

The 75%'s blockchain will always be longer then anyone else's, so would it cause all other miners' blocks to be orphaned?

If they chose to only build on their own blocks and not accept any blocks from any other miners?  Yes.

And they could essentially set rules on the transactions,

They could choose which transactions get confirmed, and which ones don't.  They could allow other miners to confirm transactions, and then they could orphan those blocks and confirm other transactions that spend the same inputs instead.  They could not create "fake" transactions, or confirm otherwise "invalid" transactions.

and set a minimum Tx fee to get a Tx included in the blockchain.

Yes, they could refuse to confirm any transaction that doesn't pay the fee they demand.

[Beliathon]

Really the only way this could ever happen is if some tech genius invents a new kind of super-processor in his garage, hides it from the world, manages to produce a few of them and get them online without anyone knowing.. oh wait that's all quite impossible nevermind.

You cannot manufacturer a computer processor without a factory...

This argument is a non-starter.

[lnternet]

Like let's say someone has 75% of the overall mining power.

That would be an awful lot of hashing power.  It would be very difficult and expensive to acquire.

Well, the hash-rate is doubling every 50 days or so. So it doesn't seem as far fetched as many make it out to be.

Just as I say that Beliathon posts his comment how it's impossible 

[skooter]

It's certainly not impossible.

If intel made an ASIC chip for SHA-256, it would make all these other chips being made right now irrelevant.