Why exactly is the Parthenon famous?

It was originally built because it was made for Athena (the goddess of wisdom), for her birthday. It was made of other pieces of marble or rock, coming from older buildings, as well as newer rock quarried in the marble quarry. That's one reason why it's famous, incorporating the old and new in one large design.

Another, it's size alone. It was one of the largest in the known world, making it one of the wonders of the world during ancient times.

Also, looking at it technically, the steps are bowed so that they look different than completely straight steps, normally found on temples or buildings in ancient Greece.

The frieze running along the top is highly decorated, incorporating a new technique of foreshortening along the edges that were small, so that standing or sitting figures could still fit, yet look 'normal'. Painted and sculpted people on horses ran along the inside of the building, having the most in any building before it. And it was the first work in ancient Greece to have as many pillars as it did.

That's everything I can remember. I might have missed one or two things, but that's pretty much everything. :)

You won't find a clear answer if you don't look. Too many kids here think this is the olde cut and paste homework shoppe.