So the NIH has decided they're going to change the rules for the K99/R00 mechanism. The Notice of Intent can be found here. The big change is the postdoctoral training application window is being decreased from 5 years to 4.
Bad move in my opinion.
As I noted on the Twits, there is no such thing as a one-size-fits-all postdoctoral experience.
I understand having an upper limit on postdoctoral training for applicants. The amount of time postdocs are spending in a holding pattern trying to land a tenure-track position has become ridiculous. An upper limit of five years is reasonable since it will give most people the chance to gain the experience they need for TT, but excludes those who's chances of obtaining such a position are dropping off due to "excessively" long* as a postdoc.
But four years? That's too short for many people. If you don't hit the ground running and pump out data rapidly in the first couple of years you will have at best one shot at applying for a K99/R00. Not all science is amenable to that. Most worthwhile science turns out not to be.
* For the record, I landed my position after more than 6 years as a postdoc/research associate.