This post speaks to the idea of assessing KPIs (Key Performance Indicators) in the context of an email marketing programme.
Any KPI / metric employed should be business focused, not operationally driven. However operational metrics will help refine the tactical delivery of the campaign, so do address the two in parallel. While "click-through rates", "open rates" (etc.) will give feedback on how the campaign performed, you must link them back to the broader, strategic, picture.
Here are some ideas for business focused KPI's
Once KPI's have been defined, then you can develop a way of measuring them to ensure that your programme & campaigns are aligned with other channels & with company goals.
Here are some more examples of KPIs*
Finally - always remember a multi-channel world means many different initiatives can influence the purchasing decision.