The LG G Flex is the most comfortable large device I've used- and has a stellar battery life. Plus a whole range of useful add ons (like being able to have an icon in the adjustable soft keys at the bottom to bring down the notification bar- no more straining to reach the pull down on a large device! (using Nova launcher is another way to address this).
The following are a couple of things that after a few months with the G flex I'd like to see in the next version:
1) Less sensitive when moving icons from screen to screen - it's super sensitive, though there is a knack- and adding new icons to a screen rather than trying to move across is a lot easier.
2) Colours are very good on the g flex- however whites do have a slight grey/blue hue (very much like the Note 2, only the g flex screen is brighter). It took a while to get used to this (and it is more noticeable under some conditions such as neon lights). After a while I found that I preferred the background for longer reading, but also that if, for example, I changed the app (eg using nitro for email rather than the inbuilt email client) there was a difference. I wouldn't change the background settings in a future g- flex but it would be nice to have the option.
3) Whilst the external speaker is better than the Note 3, it's not as good quality as the HTC One (or even the iPhone 5s). It's loud enough but you don't get the staging that you do on the HTC One (not surprisingly). On earphones the Flex is very good. For MP3 (320 and lower) and OGG formats the HTC is better on earphones (and when paired to a blue tooth speaker). However, with lossless formats (I use FLAC) the difference on headphones isn't significant).
4) For the first couple of weeks the back attracted dust. Not as much as the Xperias that I've had, but noticeable. This effect had pretty much worn off now, after a few weeks, though turning the phone over now as I write this, I can see a few bits of dust. That's something to improve.
5) Water proofing would be a nice add on (this thing feels tough, especially using an lg vest (£13) which protects the corners but doesn't add to width.
6) Ability to configure the notification drop down further would be nice. There is some configuration possible at the moment (e.g.to show/hide the q slide panel).However, you can't hide the volume and brightness slider. This fills the notification panel. I'm not sure if I would remove these as it means that the quick dials I have in the notification panel (using power toggles) are at a thumb friendly height. However, it would be nice to have the option.
7) The screen is a lot better to read in sunlight than the s4, slightly better than the htc one (the slight curve helps a bit for angling). But it's not as good as some of the Nokia's. I didn't really notice this on cloudy days, but in bright sunshine I do.