Also potentially interesting is how YC changed the VC market. [0] is an essay from PG that talks about how VC funding used to work. [1] is a short essay that starts to get at why YC is different. (These should be read with a grain of salt, since PG is the founder of YC)

TLDR is that YC was the first firm to fund a lot of small companies, as opposed to a medium amount of medium sized companies. Similar to what Square did for payments by offering credit card readers to individuals, YC offered funding to super small companies where previously it was not available.

The impact of this on the world is up for interpretation, but at least one positive result is a lower barrier of entry for entrepreneurship.

I am far from an expert so if this is inaccurate feel free to correct me.

[0]http://www.paulgraham.com/venturecapital.html [1]http://www.paulgraham.com/whyyc.html