But in the case of iOS vs Symbian:
- as others said: capacitive touch screen (this is not an OS issue, but iphone was among the firsts to use it, definitely earlier than Nokia). This is huge. Like the thing that everyone was talking about (around me) when the original iphone came out is how you could swipe to see the pictures. And it wasn't just for paging, it defined how you could interact with the phone (think pinch zooming, and rotation - not sure when these were added).
- the touch screen UI itself. Nokia played around with the touch UI before, but never really liked it. It was expressed several times internally, that touch is just a no go. But no wonder: the resistive touch screen is pretty bad, but also Symbian itself was built on the assumption that all you have is keys while iOS was built with touch UI in mind from the very beginning. (Now, of course touch was added to Symbian, but that's just not the same. Or they didn't put in the effort. Nokia even had an experimental touch phone released to the market in 2003, the 7700[1], but it was mostly ridiculous.)
- the UI just was a lot more polished, looked better, classier, the graphics was better. They had OpenGL and probably a graphics accelerator - nothing like that in Symbian, of course. (It even took the android guys by surprise, I remember reading/hearing in an interview that when they saw a demo or the release, they've realized that they had to redo the UI from scratch. Because before that they had this Blackberry-ish/Symbianish idea, they thought they were competing with that.)
- I'm pretty sure it had a better browser.
And this pretty much defines the experience, the feel a user gets from the phone. It couldn't send or receive MMS-es (some people may have used it then, but most I guess just wanted to have the feature), it couldn't receive 'push' email. I.e. you had to manually refresh your inbox, emails didn't just arrive. It didn't even have apps. Symbian had all these. It has had these for years then. It even had an app store like thing (at least you had to send in your app for verification which would then be signed by Nokia or it couldn't be installed - that was a new thing around 2004-2006, something I think nobody really did before).
