Why do I hate Nagios?

You either love or hate Nagios, there are no intermediate opinions when you use this application. I am definitely uncomfortable with Nagios. While I'm not a radical SCOM guy, because I like other free (as in freedom) monitoring systems: Zabbix, Sensu even PandoraFMS, I really hate Nagios.
And if you want there are a lot of very good SaaS alternatives too, such as New Relic, Datadog, Monits, etc.

Nagios reminds me of the good old times, when we only had a few machines in our data centers and three or four services and they were easy to manage and monitor.  Do you remember big brother monitoring software? It was quite popular 15 years ago and it was very simple because our projects were very simple comparing with our projects nowadays.