Add back button
Going backwards is an action that you do a lot on phones. Leaving a settings menu on an app, browsing through settings, browsing through folders, etc.
Most phone interfaces like on Android, Windows Phone, Ubuntu Touch, etc. have a back function that can be very easily triggered. I would like to request doing this as well and replacing the "X" button that closes apps which a back button that goes backwards. The back action is arguably needed much more frequent than the close function. The close function can also be accessed in the app overview already; Making it easy to keep track of open apps.
I do understand that most apps already allow you to go back using a button on the top left. I would argue that that is uncomfortable, as you need to move your phone inside your hand to reach that button. Personally my phone is pretty small (for todays standards), so on a more "modern" phone the problem is only going to get worse. Also it makes sense to have the back button always in the same place for easy access.