One thing that is very strong is the need for a "host" with a Nagios service definition. This doesn't map so well on to environments like Amazon EC2 auto-scaling groups. You don't necessarily know the host names in advance. You wind up building Nagios plugins that can monitor a pool of hosts (using cloudwatch or whatever) and gives you some kind of aggregate status. It sounds like a kluge to push it into the plugin, but it does allow you to use the Nagios alerting, which is pretty well understood.
