Earlier this month, Tokyo's Bunkyo Gakuin University held a seminar on doujin and indie game development. On hand was Kenta Cho, freeware shmup developer.
According to Cho, the these are the three steps why indie games end up unfinished: "No good ideas", "Got tired of it after working on it for too long", and "The game wasn't so great after all after actually starting to make it."
Cho is considered on of the best game programmers in the work. All of his games are available free of charge on his website. I interviewed him for Arcade Mania. He's a good dude.
Doujin game makers discuss doujin game scene [Canned Dogs]