It always surprises me how often people use Excel for word processing tasks. There are a number of reasons for this, many of them good.
One of the reasons is being able to organize your work in tabs. Interestingly, OneNote, Microsoft's note-taking app, has this feature. To me, that is the biggest draw of OneNote (I know there are others, and my resistance to using it probably keeps me ignorant of them ;) .)
I think other word-processing apps, such as MS-Word and Google Docs, would also benefit from having tabs. Just as with tabbed browsing, it gives you that one extra level of hierarchy to organize your work.
P.S. If outlining, a la Dave Winer, were more widely and better-implemented, then this feature would be less important.
P.P.S. One complementary feature needed: consolidate all tabs into one.