Android's back button strikes me as an interesting idea that doesn't really work. If it depends on application X doing the right thing in order for the experience using application Y to be pleasant, then it seems pretty broken.
I know you have to cheer on your team here, but the back button is completely intuitive and users like it; you're creating problems based on what is theoretically possible and not what is reality.
In the same vain, any poorly-written application on any device that doesn't conform to UI standards and guidelines will drag down the experience when using that device. This isn't a problem unique to Android, but it's more obvious when apps violate it due to how multitasking is built into the OS from the ground-up.
As for poorly-written apps dragging down the experience, I don't think that's necessarily a given. Obviously if you have a crappy app, then using that app sucks, but that crappy app shouldn't also make the experience of using other apps crappy.
Don't you think this is a silly argument to use against Android's back button when the post we are commenting on is a complaint about iOS's home button? By your logic, that should indicate that the home button's behavior is not intuitive and complicated... you know, because somebody complained about it.
Obviously some people disagree with the iPhone's home button behavior. Then again, they're not complaining that it's too complicated to understand, merely that it's not the ideal (or perhaps "correct") behavior.
Also, sometimes the back button in my browser totally sucks, such as when I click a link to a Twitter message and it decides to totally break my back button so that I have to press 3 times rapidly to go back. I don't want this experience with apps on my phone.
Would you rather your browser didn't have a back button at all?
And I would much rather have no global back button on my phone than a sometimes-broken one.
It makes sense with the way Android works. Applications are often interconnected. I can browse to reddit in Browser, click on a YouTube link and open it in the YouTube app, hit share, and open up GMail to send it to a friend. I can then hit back to go back to YouTube, then again to go back to reddit. Believe me, it feels so natural to me now that I find iOS clunky to use without it.
As for applications that get it wrong; that happens on all platforms for all kinds of different features -- it's unfortunate but it shouldn't reflex poorly on the feature itself unless it's particularly hard to implement correctly.
The "desktop" you're referring to is the functional equivalent of your web browser's home page. So yeah, the browser does take you there.
The idea that you have a linear sequence of "actions" across all apps seems pretty weak in a multi-tasking environment.
Google has simply extended the concept from web apps to regular apps -- and that's hardly a big stretch -- especially when all your regular apps can link to each other.