Mike, on the charge of being a Microsoft fanboy, you are hereby acquitted. 
While you’re trumpeting the relativity of software user acceptance (which is quite apart from my argument about system design principles) then we might also muse about how technology choices in GUI development are relative to the problems they are meant to be solving. In short, browser-based user interfaces do not suck absolutely.
If I was planning on writing the next great video editing tool, or embedded control software, or the next 3D action game, it’s clear that web browsers would not be a viable alternative in these situations - not only because of responsiveness, but because of other limitations of the target environments.
Of course, your typical web browser today is a 20MB monster with capabilities undreamt of by the first public web browsers of the early nineties. Rich GUI controls, floating dialogs, keyboard event handling, animation with synchronised sound, these are some of the things possible in Javascript inside today’s web browsers. (Find the Javascript version of Raiden II, it’s amazing.)
Google Spreadsheet probably has 80% of the Excel features that people use the most, plus it’s shared, collaboratively editable in realtime, and available anywhere there’s a web browser. It would have been impossible, let alone preferable, to build such a thing even 4 years ago. The technology changed.
Google Spreadsheets does not make sense for the corporate market. I can’t see myself ever using it wither. To be honest, I can’t think of anyone in the world who would actually find a use for this crazy idea. The point is that browser-based UIs do not suck absolutely, because tasks that were once Windows native can be made to work quite acceptably in a browser because the task doesn’t need a sophisticated GUI.
Which leads us nicely into the topic of the “power and speed” that you seem to value so highly. Tell me, would this be the same “power and speed” needed to run the Aero or XGL window managers (for Windows Vista and Linux respectively), for which a DirectX 9 graphics card is the minimum requirement? Or perhaps it is the “power and speed” with which users click on check boxes and type blindingly fast at rates of 162000 words per minute?
There is no need for “power and speed” in the GUIs of most applications because of the glut of surplus computing power that is now available. As for functionality, the trend is that the browsers are catching up to the native toolkits, and that’s without adding any Flash/Java/ActiveX/binary plugins. Hey it could plateau, but that hasn’t happened yet.
But I agree about the WYSIWYG part. 
It would be nice if all blogs did this, but instead of waiting for each of them to improve I can see the attraction of… err… one ring to rule them all. And that’s where the Microsoft-taking-over-the-web conspiracy comes in. (And you naysayers thought it wasn’t real!)