Mostly stability. I've yet to own an Android phone that the dialler hasn't crashed either placing or during a call. And I've owned around a dozen different Android phones since 2010. I've had issues with GPS being wildly inaccurate, screens losing touch sensitivity, random force close events in Google and third party apps, terrible camera software, WiFi working sporadically or not at all, sound server crashing in the middle of calls (Nexus 4 is notorious for this), and so on. All of this across the gamut of cheap to flagship phones running AOSP, Cyanogen, and carrier/manufacturer ROMs from 1.5 through 5.1.
There was a time when I wouldn't touch an iPhone with a ten foot pole after using the original and a 4S, but with the 6 and iOS 8/9 Apple has finally made a phone that surpasses Android and Windows on the balance of stability and function.
Very strange. I too have owned multiple Android devices but never faced so much issues on branded mobiles. Never seen dialler crash even on cheap mobile. Experienced random crash on a very early very cheap tablet. I did lose touch sensitively(not fully but a narrow strip) on N4. Which resulted in me purchasing One Plus X. The mid range mobiles are now very good. iPhone was always out of question for me prices are too high here.
There was a time when I wouldn't touch an iPhone with a ten foot pole after using the original and a 4S, but with the 6 and iOS 8/9 Apple has finally made a phone that surpasses Android and Windows on the balance of stability and function.