The 2 day part is just a perk, but honestly even if I were paying for the normal 7-10 day shipping it'd come out to more than $100 a year just from the textbooks and general things I order.
It's not just free 2day, it's also $2 overnight shipping! That's the only reason I have prime. All the other crap, such as streaming movies I don't want to watch is just useless fluff. $100/yr for free 2day or $2 overnight. That's what makes amazon.com for me. If it weren't for this, I probably would not buy from the site.
Also, you get to give out free prime memberships to your family members.