Battery life
A big screen means you'll need some power to keep it lit up, not to mention to service the quad-core processor as it zooms along. All this on an Android phone - an operating system which is known for running programs in the background and reducing battery life.
But the battery is not removable, so the space is taken up with minimum packaging and maximum juice. And there's a Stamina Battery mode to eke out the most power. Turn on this mode and a lot of the phone shuts down: WiFi, Bluetooth and mobile data are disabled, though calls and texts will still come through and any software that's mid-download will finish before the data connection cuts out. Alarms will still work, too. These changes happen as soon as the screen turns off, and are re-enabled when you start using the phone again. How much does it help? Well, if the calculations on-screen are accurate, it can boost the remaining standby time from eight hours to two-and-a-half days. Whatever the exact figures, it certainly helps.
Even without Battery Stamina, the Xperia Z seems to last a full day happily, though for peace of mind I'd still recommend nightly charges.