People can copy what you've done, but they can't copy what you still want to do.
Stop sketching and start building.
I'm still a really shitty programmer, but I know enough to hack a prototype together.
One of the biggest hurdles about Foursquare is you need to remember to use it.
Don't let people tell you your ideas won't work. If you're passionate about an idea that's stuck in your head, find a way to build it so you can prove to yourself that it doesn't work.
Building a product is easy. But building the company that builds the product is hard.