Product feature metrics - why are they important?

Measuring the success of a new release is tough- and it's necessary. The PM teams I have been a part of are always trying to figure out new ways to measure product release success. How do you measure success?

A few of the ways I've measured it in the past have been trial to customer conversion rate, retention rate, renewal rate, support calls and customer feedback gathered through surveys. I don't think there is just one way or even 10 ways to measure success, but I know that it's important. Are there tools out there to measured by success of product releases? What are you using?

Ive been lucky to be part of teams that have released new products and bug fixes, and have also done minor releases. Each time, success is measured differently. If I implement a minor feature, success is more difficult to measure- and without A/B testing, how can you be sure that the release was effective.

I'm going to take a bit of time and look at some tools to measure this - any advice or guidance?