One thing it will change is reading. The iPhone 4 is very good for reading despite being small, and I think it can only be the absurdly high resolution the screen has.
And of course many other things, photos, art, art books, and comics can benefit a lot, if the files are delivered in high enough resolution.
On a Mac or PC, the interface elements get smaller with increased resolution. This is not true of the iOS, that's why it can be done without disadvantage.