Re: OS vs apps: Paint
Given the price of iphones, it's odd to claim credit based on it being cheaper...
The original iphone wasn't a smartphone (couldn't do apps) other than the "it's marketed as a smartphone" definition, it did however do Internet access as was standard on all phones by the 2000s. If networks unfairly gave iphone users cheaper data plans, then that's something to criticise.
"hugely successful" - 8 million or so isn't bad for a first release, but it's way down the list on best selling phones. For comparison, the 2009 Nokia 5230, the best selling smartphone of all time, sold 150 million. Also Apple put all their weight behind one phone - companies like Nokia and Samsung vastly outsold them (and still do outsell them, though Samsung overtook Nokia).
GUIs are a matter of preference, I don't find anything "slick" about it - you touch an icon, it does something, nothing new. Multitouch can be useful, but no good for one-handed use (amusingly the standard argument iphone users make against Android - well, until iphone 6 that is). That leaves us with minor stuff like scroll bounceback (which I always found annoying). Handwriting recognition was just one of many things that other people were looking at and supporting, as indeed they still do (Galaxy Note, and Windows devices). Various companies have introduced swipe-based UIs since then, this wasn't all Apple, and I'm not really a fan of those actions anyway.
Of course, I'm sure the Apple fans will pick up all sorts of minor reasons about what was special, because we constantly have to hear about this 7 years later. I'll thank Apple when I hear them thanking the many innovations they take for granted that came first from other companies, both before and after.