Dan,
What I've been seeing is a sort of 'Deja-Vu' with Web apps. With Desktop apps, we used to hear about all sorts of compatibility issues--versioning, setup, OS differences, etc. Then here came Web development tools & languages, most notably Java, which was supposed to solve all that-- "Write once, run everywhere" was the mantra we heard. Well, as I alluded in my first post, that was very far from reality--we've all seen our browsers return messages like "You must be using IE 5.5 or greater to use this" or "Java Script Error--Java not installed" or "The nnnn Plug-in is not installed", etc. etc.
So we're back to the old days of incompatibilities and setup issues--nothing's changed, except that we've lost flexibility, speed, etc. Sheesh, in C++, you could set the compiler to compile for an Alpha, ix86, OS400, and a few other hardware platforms--in my opinion that was alot closer to 'write once run anywhere' than we are now. And all the new 'plug-ins', browser versions, etc, all seemed to be aimed at gaining back the flexibilty and all that stuff, and when it's all said and done, we'll be right back to where we were with desktop apps--flexibility & speed at the price of lost universal compatibility.
And of course what I consider the single most annoying and constraining trait of all--the Single Document Interface of browser apps--I don't see that changing anytime soon--but somewhere someone will come up with a 'standard' for special 'control' to emulate the familiar and highly useful Multiple-document interface common to most Windows apps. And then people will gush that it's such great 'new' technology--but it will only run on, say IE 9.0. Again--back to 1990--we had MDI apps, flexible and all, but only on Windows.
So, in summation, I think that quite obviously the Web is a huge advance and hugely important for so much of what it is used for, but it should never replace standard Windows (or Mac or *nix or whatever) Desktop apps as far as custom applications go.
--Jim