Mike Taulty posted a great piece on the Spectrum of Richness an Reach which talks about and shows how the different technology platforms we have differ in terms of their power vs cost.
I've put together something similar for Knowledge of both users and developers.
I think HTML is the easiest for developers to use (think back to the 90's every man and his dog was writing HTML) and that the web is the most comfortable for 99% of users out there, they know and understand the web and how to use it, Windows Applications are much less comfortable to most.
Not sure it's 100% accurate but as a Web Developer it's certainly how I see it.
What do you think?
P.S. I'm loving Office 2007's drawing features.