My problem with what stuff to finish is almost the opposite. I know how to make simple games, have made Pong, Snake and a really simple shmup, but I really wish I can come up with more original game mechanics. Plus I prefer to work more on the "inner" workings of a game, especially graphics engines. I am in the process of making a 3D engine, but I don't know how I feel about using it for just a fancy 3D Snake or shmup if you get what I mean. I don't want to finish most games I start simply because I don't find them novel enough.
Maybe as game developers we sometimes force ourselves too much to be good game designers, even though it's not always necessary to be one. If I release a game to the public, I personally think that I'm expected to make a very fun game, even though I can't come up with many original game ideas.
Currently I am making a puzzle game clone, and in fact I will incorporate the 3D engine in some way, even if it's just something not directly important to the game, like providing backgrounds. Still, it's a clone of an existing game. But I think I could finish it this time, because I am incorporating the graphics engine (something I am putting more effort in), into the game for the sake of a more complete, polished product. It's turning out to be a good challenge, because I can see a real world application of the engine instead of being just a show piece. I am figuring out how to make it more adaptable for this game in particular and how to load it as a library without a fuss.
Basically, my graphics engine is my main passion, and now I am encouraging myself put it to work in a real-world situation. It will help me get through making this game from start to finish, even if the concept has been done many times before.