As of this writing...

A WSJ Bitcoin post from today has 373 points, 207 comments, was posted 10 hours ago, and is on the homepage.

This other post "Reconsider" from today has 558 points, 117 comments, was posted 5 hours ago, and is __not__ on the homepage (anymore).

Source: http://imgur.com/a/vNGLm

Question for all: what determines lifespan of a post on HN homepage?