Until we get real high speed internet, 100 Mb or better, the web will not really come into it's own as a potential replacement for the desktop. Until I can get anything off the web as fast as off of my hard drive it's not happening. I love the web and and I love the desktop. Both have their strong and weak points.
Even when you edit a 500-slide powerpoint, all you need at any given time is to see the current slide. Same for word processing.
Rather, I agree with the original poster that it's the APIs that are the weakest link. HTML and CSS are ugly APIs when you think of GUI development.