I just read an interesting post on Sweating the small stuff by one of the developers of Jottit.

Even though Jotit does not use any fancy Ajax techniques any developer who has done cross browser web application development will understand (and feel the pain) of this quote:

And when you start sweating the small stuff, it's frankly incredible just how much of it there is. Even our trivial site is made up of over two dozen different screens. Each one of those screens has to be designed to look and work just right on a wide variety of browsers, with a wide variety of text in them.

In my experience the last 10% of effort  are what make applications a pleasure to use .