I did a pretty long (and I think good) writeup of Engagement Metrics over at The Analytics Guru Engagement Metrics Methodology and Measurement where I actually saw where this is going - to a standardized set to tags against an agreed upon industry standards of how Engagement is going to be defined and measured.

When you think about it - why not have Web Analysts who work with Brand Marketers - who cooked up most of the terminology of what they want to be measured - embedding tagging into a campaign (along a predefined way to measure  if visitors are engaged) and then comes the measurement of it.

Anyway, I talk more about taking it up a notch by adding the customized engagement tagging in Engagement Metrics Methodology and Measurement.