Marco’s recent experience using a 6 Plus reminds me what still annoys me about its smaller sibling.
The battery is too small when traveling. Most of the time the battery life is just fine, but whenever I am traveling, and using the phone very intensely to check maps, track my steps, get information, and take pictures, I have to restrict my usage to make it through the day. Part of the problem is that the automatic dimming sets the screen brightness very high outdoors, but otherwise we just need a larger battery.
The case is too slippery. Mainly because you no longer can properly grip the phone, it now becomes mandatory to use a case to get enough friction to have the phone not slip from your hands.
The keyboard is too large for one handed use. The 6 works remarkably well one-handed, provided the back is not slippery thanks to a case, apart from the problems with typing. Here we would really need a narrower keyboard, the size of the 5 keyboard shifted to the side of your thumb. This is a hard design problem, because then the phone would ideally need to figure out how you grip your phone to present you with the correct layout. But maybe we could detect a sideways swipe on the space bar to quickly move the keyboard around?
In general I would like to see the next iPhone prioritize whole day battery life while traveling above extra thinness.