I think for every delicious side project that succeeds "in spite of itself" there are 10 side projects who don't fulfill their destiny of greatness -- because they're missing the dedication ingredient that most startups require.
It would seem to me that the biggest missing piece for most startups is making something people want. I guess you could say dedication is what's missing if that's what you need to go from something people don't want to something they do want. But how far is someone willing to steer away from their dream idea of Facebook for Dogs? And how able are they to actually figure out a problem to solve? My guess is: not very.
I think you've nailed the toughest part about working on projects in general, not just side projects - determining whether the thing isn't working or just needs more commitment and time.
So true. Dedication isn't a guarantee; it's more like table stakes. Maybe the trick is to be dedicated to an outcome, not a particular idea.