Much fun as it is to think otherwise, Mt. Gox is already an example of a decentralized exchange. What that means is its not the only exchange to trade BTC.

Just because it has become the largest exchange means BTC traders like something about it.
"decentralization" just means to 'unlike' Mt Gox and trade elsewhere.

What you are asking for is a distributed exchange. Now, If you have millions of brokers. You have millions of points of failure.

Who is going to enforce settlement of trades?

Still not sold on the distributed exchange idea.