I almost developed a content platform / niche specific writing community for which there was a huge demand and ready made user base at my disposal and decided against. That was in 2011. I had an awesome domain for it, which I sold.
I've now come back to this project, repurchased the exact same domain (fair price) and will be building that platform with a 2017 launch.
I knew it would work in 2011, I know it will work now in 2016, only it would be a lot easier to establish in 2011 and I'd have a massive business right about now. There is still a decent ready to go userbase for me to go and grab, only its shrunk significantly.
At the time I was well connected online and respected within a specific niche community / subculture and its only with hindsight that I realise how valuable it is to have a big network to tap into when it comes to getting a startup off the ground.
So I've got the chance to undo my mistake, but I still regret it.
On the plus side, I've been busy liquidating my domain portfolio and got back into developing web properties, at a time when there is a very healthy market for selling established sites.
That has been a natural progression from domaining, after domaining got me back into the game and interested again (pretty much after just collecting a few domains that I simply don't want to part with, and now have a hunger to develop). I built a nice little empire on the net back in the day which got destroyed by some algorithm updates, it left me with depression and no desire to build anything for 4 years, now I'm building again and I thank dabbling in domains again for getting the fire back into my belly.
I'm enjoying the internet again.