My experience of the Nexus 4 is not really anything like yours and with all the things you think are weak/wrong with it I do have to ask why you bought it ?
A stop-gap once I realised that htc desire for some reason became a little unreliable, and the fact I need ££££ for other things. It's a shame because I planned to wait till the end of the year and buy LTE.
No LTE. Well in a country without a 4G network and that fails miserably to even have good 3G coverage in many places I don't see that as a drawback.
This is TODAY. 4G auction is taking place in March and by the end of the year there should be coverage. I take it as a clear sign that 3G won't be expanded any further so 4G is the only chance for the countryside connectivity
Yes that glass back shatters if you look at it the wrong way, well not really. I've managed to not break mine in the 2 months I've had it and that includes dropping it a few times. Funnily enough I heard the same scare stories about the glass back on the iPhone 4.
Yes both 4's are more similar than different. You can count yourself lucky. xda developers and youtube videos reveal a lot of sad stories.
Weak 3G and Wi-Fi. Nope haven't noticed that either, just the standard rubbish Vodafone data signal in Edinburgh and that is the same regardless of the phone or OS in my experience.
the signal is rubbish, which is where strong antennae can be a life saver (literally). I am not aware which phones are the best, but in my experience my every upgrade was worse and worse in terms of being able to make phone calls from the countryside or my bed (not a well connected place)! In SK and Japan they have no idea what bad signal means and therefore don't care.
It also doesn't help to have wifi downstairs either. At -70db mac mini and MBP connect just fine, but n4 and ipad only see -80db and really struggle
The whole point of the Nexus phones is that you get the stock Android build, that way you don't get all the rubbish that various companies shove on top making it potentially slow and clunky. It might not be exciting but it's clean and it's very easy to add apps and widgets if you want the look of something like Samsung's SIII.
It is entirely true, which is why I bought it. But let's not forget custom ROMs, and the fact android asks to be rooted to get rid of adds (/etc/hosts) and at least access DSP manager
The other advantage of the Nexus devices is that you get the latest Android version first and can look forward to the 'excitement' of Android 5 in a few months.
The screen as you say is not over-saturated and in comparison to other phones including the SIII is in my opinion more natural looking.
I've never said this was bad. I am yet to compare it to ipad and my monitor for color accuracy. SIII looks fake, but they also make it easy to "tone it down"
The choice and quality of apps is one of those Canon/Nikon arguments and is very much dependant I think on what apps you want and use. I'm sure there are some in both App stores that aren't in the other and some that are better on one OS than the other.
I've used plenty of SIIIs and in my opinion they are no better than the N4 and certainly not worth the extra money.
LTE (if you get the right one) and no glass back are the main advantages
As for whether the OP should swap from his iPhone, well that's really down to whether you want to spending all that money.