What are some of the key metrics you'd track as a Product Manager had you been working in a SaaS startup? How would this be different for a mature SaaS company?

Interview question asked to Product Managers interviewing at Microsoft, MongoDB, Benefitfocus and others: What are some of the key metrics you'd track as a Product Manager had you been working in a SaaS startup? How would this be different for a mature SaaS company?.