Most of the aforementioned metrics are interesting but won't help you improve team performance. Problem is your asking a management question in a development forum.
Here are a few metrics: Estimates/vs/actuals at the project schedule level and personal level (see previous link to Joel's Evidence-based method), % defects removed at release (see my blog: http://redrockresearch.org/?p=58), Scope creep/month, and overall productivity rating (Putnam's productivity index). Also, developers bandwidth is good to measure.
