High information density
The iPhone screen is small. I keep using Sparrow, despite no longer being in development, because it allows me view a decent number of emails and their previews on the screen.
I've found other email apps, including the defailt Mail.app, to waste more screen estate than I would like.
I would especially welcome view options in the settings to choose how many lines are displayed in previews, so that users have some control over how the screen estate is used.