Quick summary, since the site is down:

In response (obviously) to http://www.slideshare.net/superdupersheep/stop-using-nagios-... (also available through HN).

Author uses Nagios at Etsy, with 10,000 checks (mostly in the 2 minute range), and it seems to work well for them with some minor tweaking. They have a plugin that provides a REST API. And he thinks the rest of the complaints are about backend stuff that he doesn't deal with much or at all (configs, wire formats, et al.)

He considers Nagios to be simpler than the proposed Sensu, and prefers "Unix style" applications rather than monolithic ones, where Nagios is certainly on the simpler side.

So he will continue using Nagios because it works well for his use, and good luck making something better.