Is it? At the end of the day, that's what the web is, right? Gmail (or even Google Docs) may or may not be your favorite email client, but I don't think it's necessarily a bad idea, per se.
I think you're right about user expectations on mobile. I also agree that we've all seen how terrible the UIs on some cross-platform Java and Flash applications are, but—given the success of the web on the desktop—I'm not sure it's fair to condemn cross-platform UIs on the desktop.
I agree that the web is the exception to this rule (see http://www.springenwerk.com/2011/09/thoughts-on-mobile-ui-de...). I quote: "Surprisingly, cross-platform UI works on the web. People are used to it. People don't think it's odd that GMail doesn't look like Outlook or Mail.app. They don't complain that the buttons on Twitter look different from the button of the other apps they use. They are able to use Google+ and Facebook although the UI looks "foreign" in comparison to other apps on their platform. Strangely enough, all those things that make cross-platform UI a bad idea seem to be absolutely OK on the web. Why is that? It has to do with expectation. When users open their web browser, they know that they are entering a very diverse space."
It's probably better to think of the browser as another platform. In the sense that users have specific expectations about how it behaves, what it can be used for and how to use it.
Attempting to build a UI shared between, say, Windows and the browser would be a bad idea.
Is it? At the end of the day, that's what the web is, right? Gmail (or even Google Docs) may or may not be your favorite email client, but I don't think it's necessarily a bad idea, per se.
I think you're right about user expectations on mobile. I also agree that we've all seen how terrible the UIs on some cross-platform Java and Flash applications are, but—given the success of the web on the desktop—I'm not sure it's fair to condemn cross-platform UIs on the desktop.