Grafitti was revolutionary back in the day. So was Apple's Newton Handwriting Recognition. But that was a LONG time ago!
What I now cannot live without is stuff like my BlackBerry automatically capitalizing the first letter after a period, or the first letter of a new paragraph. Adding in punctuation for me is also key. I type "dont" and it becomes "don't" without my having to worry about it. Still another phenomenal BB feature is Auto Text. I type "blb" and my Tour makes it "BlackBerry". Once you get to rely on these shortcuts, going back to any other device is pure hell.