I've always thought that Nagios configuration was pretty good. For the small organizations, it's simple enough with it's object/template infrastructure, particularly now that you can create custom properties and pass arguments to nrpe, that most sysadmins can get a monitored environment up in 30 minutes, and then add devices on the order or 2-3 minutes per system (inheriting all the monitors for that object class)
When organizations get larger, the configs are straightforward enough that the machine generation (which is absolutely necessary when your monitoring 10s of thousands of services) is also relatively straightforward.
My company is dropping north of $1mm on ITIL practices, ticketing, trending, and KPI generators to depart from our current Jira/Cacti/Nagios world, but Nagios is one of the products that we're continuing to use for the foreseeable future. (Eventually that will change - I've discovered that Billion Dollar corporations would rather spend $500K / framework and then start adding developers (at $150K fully loaded) to extend that platform with proprietary in-house developed systems, rather than simply go buy an open-source product for $5K and do the same thing.
It may just be that the Nagios developers have a different cognitive style than I do, because I find the configuration language to be irritating to the extreme. So much so that I end up using templates to script configuration as much as possible. Somehow monit has always been more comfortable to use in my hand.
Interesting. About the only "Nagios Annoyance" that I have (and I don't really see a good way around it) is that you don't specify the commands you are running for a service check, but have to reference your command library (which in turn has the name and calling order of the actual parameters)
I realize that there is no way to update the command/parameters you are calling in one place without doing this, but the cmd entry usually almost _completely_ maps to the command that I'm calling - parameters, command name, etc.. that it's addition has never really seemed to be that critical - and always results in one more "lookup" when trying to figure out what a service check is doing.
Anything that improves Nagios configuration is a good thing.