Key metrics for a customer support and success community

A few that come to mind:

• Support ticket deflections. Does someone contact your support team less when they find and engage in the community? You can also combine this with average cost/ticket for your team and show that the community is driving cost reductions.
• Time to answer. How long, on average, does it take for a user to get a support request answered in the community? You want to make sure people are getting their answers in a reasonable time, otherwise they're going to be frustrated and contact support anyways.
• % of support requests answered by community vs support team.