The people... well I guess the community aspect of the site. There's a good chance you may know someone or know someone who knows someone who wrote a review on an item if you want to ask a question about it. Makes it alot easier to choose the right ones the first time.
So many things. Earning points, the compare options, being able to narrow down searches by size/texture/material etc., the community, all the reviews. I feel like it's a lot easier to make an informed decision buying something from this site than from some other sites.
The compare option and wishlists. I also really love the incredibly detailed product descriptions.