Why you should build an application (even if it already exists)
Well done post.  You could find a reason not to do anything.  Google? Check out the search engines that already exist and own the space.  Apple? No one wants a computer for personal use.  Dropbox? File storage has been done and no margins.  Twitter? 140 characters is worthless. Vizio? Enough TVs exist.  YCombinator? WTF are you going to do with $15k. 
I could go on and on and on.  Seriously, just fucking build it.  If for some odd reason NOTHING at all exists like what you're doing, a competitor will come along that does.  There is nothing and I mean nothing that a smart team of 20 somethings can't build nowadays.  At worst, you'll have something that you, yourself love.  That's the key to all of this: Build something YOU want, then adapt it so it works for a broader range of people if need be.  If the tshirt for getting accepted to ycombinator is: build something people want and the tshirt for getting acquired is: i built something people want, the tshirt for getting asked to interview/demo should be: i built something I wanted.