There is a dark time in WordPress development history, a lost year. Version 2.0 was released on December 31st, 2005, and version 2.1 came out on January 22nd, 2007.
[Matt, Founder of WordPress]
What killed us was “One more thing.” We could have easily done three major releases that year if we had drawn a line in the sand, said “Finished,” and shipped the darn thing.
The problem is that the longer it’s been since your last release the more pressure and anticipation there is, so you’re more likely to try to slip in just one more thing or a fix that will make a feature really shine.
It’s even worse because development doesn’t happen in a vacuum – if you have a halfway decent idea, you can be sure that there are two or three teams somewhere in the world that independently came up with it and are working on the same thing, or something you haven’t even imagined that disrupts the market you’re working in.