I’ve long been a fan of Stephen P. Andersons work, especially his presentations. One diagram that really stuck with me is his User Experience pyramid. I’ve reproduced it here using Omnigraffle, but you can find the original in his Creating Pleasurable Interfaces presentation on slideshare (slide 15).
This diagram emphasises that products can be beautiful & usable but still lacking in meaning. Anderson also points out, toward the end, that applications like Craigslist have achieved meaning without ever being beautiful.
If you ever think your application is finished and your work is done, have a look through the pyramid. There’s always more to do.