Some mail services, including Exchange, support read receipts and delivery confirmations. When using one of these services, it would be nice if iOS Mail let you take advantage of these features.
Of course, if you still don't like Mail in iOS 8, you can choose to instead use a third-party email app. In that case, your #1 feature request will be for Apple to—finally—let you set that third-party app as your default email client, so that common email actions, systemwide, will use that app.
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