One of the most useful features of Android, in my opinion, is spell check. When attempting to convey information quickly, it's easy to make mistakes, and it's nice to have a device that catches them for you.
With Ice Cream Sandwich, users can expect a revamped keyboard, inline spell check capabilities, and improved copy/paste functionality.
The new keyboard doesn't look too different from the Gingerbread keyboard, save for its new color scheme and speech-to-text button. The way it performs, however, has seen significant improvement in terms of spell check, and copy/paste functionality.
Spell check now allows users to click on a questionable word, displaying a list of suggestions, or the option to add the word to the device's dictionary. This isn't too different from Android's current functionality, except that it adds a bit of polish to the UI, and word suggestions have supposedly been improved.
As for copy/paste functionality, Google has made it "consistent across all of Android," meaning there's no more guesswork as to which apps will allow for copy/paste, or how that functionality will compare to other apps (ahem - SMS app). Users can also highlight and drag words or bits of text to reposition them.
I have to reiterate that while these are relatively minor tweaks, they are hugely important in solidifying Android as a cohesive OS, and giving Ice Cream Sandwich a smoother, more streamlined user experience.