In my humble opinion we have reached a point with phones where the screens work in al conditions, they are more than fast enough for most tasks and there is an app for everything. The cameras are way better than standalone compact cameras from not so long ago and the sound quality is easily acceptable for music playback.
And then there is battery life, a continual and increasingly loud cause of complaint. As all of the other problems have been fixed, and perhaps because of this, battery life has remained somewhat stagnant. There are phones out there that offer decent power times and of course many still offer replaceable batteries, but we remain in a time where a charge a day is in order to keep things working, or in my case twice a day.
I use a portable charger for when things get tight around 2pm and a 20 minute surge is usually enough to get me through the day, but there are still times when I run out too early. A trip involving lots of photos, GPS use when finding locations while walking and even general use when the signal is not as strong as would be ideal. All of any of these can knock the battery quickly and so I remain aware that a charge will be needed at some point.
I see power as the last great fix for phones and when a manufacturer can offer 3-4 days of standard use, or 2 of heavy use, we will be at a point where the phone is even more useful than it is now. For me, the iPhone SE is much more useful than the iPhone 6 because of its size and better battery performance would feel even more useful.
It would be enough for me not to have to worry and to truly feel that my phone is disconnected from everything, and that the holy grail at this time. Just maybe that will happen within 12 months and something tells me that could be a reality soon.