I've never put "Thin" at the top of my priority list for iPhones, since Apples obsession has always assured us anything they build will be slight in nature.
Funny this topics has come up, since upon unpacking the iPhone 5, I marveled at it's profile. While I'd read all the rumors & knew it would be like a waife, It's hard to imagine till you have it in hand.
Perhaps a 3mm thin see through model will be Apples stopping point.
It will literally be too thin if it causes the phone to break/bend easily. Outside of that, personal preference. I find the phone to be a bit too thin for my taste as it currently sits, though nothing that a case, or some time getting used to it, wouldn't fix.
The 5 is almost too thin for my personal preference.
Any more thin than the 5, and that is nonsense!
I prefer the thickness of my 4S. The 5 feels crazy in your hand. So thin and light, especially if you're used to a 4S. But there is a frail feeling about it that I dislike, probably because I am used to my glass 4S.
Either way I'm going to have to deal with it when I upgrade and get the 5S. And I'm sure it's something that I would get used to, just like anyone else. But I really do appreciate my 4S build/weight.
Every year I think that any thinner and it would be too thin, so far I haven't had an iPhone feel "too" thin. I think an even bigger issue is too light, heck, I almost dropped the iPhone 5 because I expected it to be heavier. Of course, I always slap a case on the phone, so the 5 could very well feel too thin.