Unusually long development time
No joke, I installed the open-beta/pre-release years ago, played for a bit, and uninstalled it. When the actual release dropped, I had the most intense déjà vu about it all because I forgot that had even happened. I had to go back to my Steam library to puzzle it all back together.
As far as labors of love go, Stardew Valley is probably the most current example. People paid for this thing years ago, but Concerned Ape keeps adding new features anyway. The retro graphics give this thing a timeless quality out of the box, so it already looks "dated" - this hasn't stopped the robust player community around it. We'll probably see this game stay relevant for a long time.