You are building a business. It does not spring from your forehead like Athena, or get pooped out of your pet Nibbler like Dark Matter on Futurama. Listen to what everyone else here has to say. Sure, pick something with favorable long-tail SEO dynamics. Sure, pick something with a viral loop. Sure, build yourself a tribe.
But then, for god's sake, pick something you can stick with, nurture, protect, and grow over the long run. That thing you don't have, that you keep calling "a fucking great idea"? Most of us call it "a winning lottery ticket". Stop thinking about playing the lottery. Get back to work.
FogCreek was a wee little consultancy which got the market shot out from under it which launched a product that nobody cared about which happened to produce an industrial biproduct which, ten years later, employs a few dozen developers in an office next door to the New York Stock Exchange.
My software -- which is not nearly on the scale of either of the above businesses -- launched to all of 116 visitors. Four and a half years of very, very part-time work later, it pays my salary. (It will sell the hundred thousandth dollar worth of software some time before Halloween.) I'm happy as a clam and getting the next adventure ready as we speak.
Totally agree with you point though.