To my ownself, the answer is simple:
1) I don't have time to tweak or customize the phone already. Nearly everything is default (wallpaper, font, etc.). So I don't need to root the device or have time to play around those things anymore, like Kit Kat, Ferrero Rocher, Van Houten, Godiva, etc.

2) I want a stable phone with stable operating system (OS) and software application for work and own usage. This include as low risk virus problem as well. I don't want to reboot my phone from time to time, or crash, or freeze, etc.

3) I don't need high-end processor, for that, I'll rather use my i7-4770 + 24GB RAM PC, or the xeon servers.
Whether a lot of memory or not, current iPhone 5 memory is fast and sufficient enough for me.
For super read/write, I have fusion i/o card and ssd in computer to play around.
For games, I use iPad. Or my trusty PS3.

4) Display - 5.5" is a welcome for me now. I don't need super whatever crap resolution or full HD or whatever, current iPhone 5 display resolution is good enough for me.

5) Camera - 8Mp is more than enough. I prefer 5-6Mp only, smaller file size taking up space. If I need anything higher or better, I'll pull out point-and-shoot or DSLR camera. Most of the pics taken using phone are for casual use only.

6) NFC - what ? Is there anyway we can use it in M'sia yet ?

7) so why I'm going to upgrade ?
(i) My 2 years iPhone 5 contract is coming to and end. Why not get a subsidise phone with contract than not getting anything ?
(ii) Company pay my phone bills, so I have option to take more expensive plans to subsidise the phone.
(iii) Got the budget to change phone.