Provide packages, please, not arbitrary tarballs from s3.
Besides a well-behaved package being automatically uninstallable, what's the difference? Either way you're running code that you only trust because of where you got it from, rather than because you've read it and understand it.
Packages are also easier to audit and deploy on internal networks than an install script which can change at whim.
In short, I have no guarantee that the bash script or tarball I'm installing via the piped bash method is the script or tarball the authors intended to be installed. I also have no way of installing this on servers without internet access, short of hacking the install script (of course, this matters less since this is monitoring as a service, but as a general principle it still stands).
Provide packages, please, not arbitrary tarballs from s3.
With regards to the MySQL plugin, I don't see any checks for active threads versus max threads, deadlocks warnings, InnoDB status warnings, replication status, long running queries, etc. For actual host monitoring, I don't see anything about load, disk usage, logged in users, memory usage, processor usage...
In short, I don't see anything particularly useful if you're actually monitoring a MySQL host.
Sorry if this seems abrupt, but if you want someone to pay for a Nagios replacement, it has to be an actual replacement for Nagios, which this does not actually appear to be.