What are some important MySQL status variables to keep an eye on?
Here's a link to a bunch of links on "tuning best practices."

The short form is: It depends. What are you tuning for? Tuning read performance is different than write performance and everything changes when you move to a cluster. There is no one set of "magic variables" that can be quietly optimized for every situation.

From these, you can look at monitoring variables, as the variables that need to be tuned are strongly related to the variables that require monitoring.