It's a combination of things. Firstly, the screen feels cheap; plasticky. The previous generations had much sturdier glass that, to the touch, just felt 'better' in terms of quality. I've not treated my 3 as well as I should have, and I've never used a screen protector, but a year and a half down the line it's still in pristine condition. I have very little faith that the air's glass could weather the same. And no, I'm not stabbing my fingers at the screen with any more force than is necessary, but when playing something that requires constant thumb engagement, excitement can result in excess skin-to-device pressure. 'Then', the bend becomes worryingly apparent.
Then there's the hollow tapping sounds when typing. True, that's more of an irritation than anything, but it's detrimental to the overall experience.
Thirdly is the 'air' gap between the screen and the glass. It's not much, granted, but it's enough to shatter the illusion that the glass 'is' the screen. Instead you're made very aware that it's just a barrier between you and whatever exciting things are happening underneath.
I've tried to like it; buying a four has left a sour taste in my mouth for all of the logical reasons. It's even sat on my desk still fully boxed! Maybe I'll have a change of heart and return the 4 still, but I'm not holding my breath.