If you could choose one feature for iPhone OS 4.0....
For me it would be a better way to organize my apps. The current screen flicking just doesn't work with 100+ apps. I can't remember the names of things and can't label the screens to group apps in a quick way to find them.
I purge apps from my iPhone because I just can't let them take up the real estate (I don't use them often enough) and that's a shame. I know Apple is going to frown on folders -- but couldn't that be an option for people who want them?