Congrats on the Christmas present. I'm the opposite- I have an iPad but have never ad an iPhone and am getting the 4S as a Christmas present.
Some of its best features:
It's screen size. I have a smartphone and just being able to access apps on a screen this size is amazing. Makes browsing the web, reading news, watching movies etc really great.
Portability. I have a bulky laptop with no battery life any more, so this is really nice to carry around instead.
Speed. Love the quick start up time and how smooth and fast iOS is.
The gestures that come with iOS5: pinch to close an app, swipe up to see task bar, swipe across to move between open apps.
And of course, a lot of individual apps that are awesome.