The back button was also the most jarring thing for me when I first got an android phone, from memory. The main irritation was when you hit back several times to "close" apps in a chain... until you got the browser, when suddenly it starts going back through your page history. Maddeningly inconsistent.
I do like the (otherwise) universality of it now though.
The inconsistency still infuriates me, even after years.
In most apps the back-button will take you back one screen or close the app when you're on the "app home" screen. This overload makes it difficult to predict which of the two will actually happen. And the penalty of a mistap (app closes, has to be reopened) is big enough to turn it into a permanent annoyance. I'm closing apps by accident all the time.
And then don't get me started on the apps that break with the pattern altogether (browser and many games) which have to be exited with the "home"-key or some menu function.
I do like the (otherwise) universality of it now though.