The easy answer is probably completion, but when I really think about it, you see the end coming from far away. And its not always satisfying to say "I'm done, this is good enough for version 1".
There's always a hitch or hang-up in a project, something that makes you hit the books or the internet for a few hours and find a solution. When you do, those moments are truly satisfying. I've stood up and high-fived myself a few times on problems that really got me stuck.