Ability to apply new filter to existing email in inbox... like Gmail does.
Universal rule: if you wish to beat the competition, you must at least provide what they do (or make the feature obsolete). Otherwise you are by definition lacking and inferior in the way of lacking.
In this example: The ability to apply new filters to existing emails in inbox.
Having to manually filter the email in my inbox because the new filter only applies to NEW emails ENTERING the inbox is a waste of my time, one which I'm not burdened with on Gmail.