As a general rule, it’s best to finish what you start.
(That, of course, puts a high premium on choosing what to start and when it’s going to finish. Most of us discount both of those pretty heavily.)
But there are two big exceptions:
First, projects that intentionally never finish. A daily blog, pushup regimen, or yoga practice is a good example. The point isn’t to touch your toes once; it’s to become more flexible over the years.
Second, projects begun on behalf of the community and designed to devolve. You can’t delegate a daily blog post, but you could start a movement that others can lead if it turns out to be popular.
What needs to finish? And what would you like to start?