Most of the default interfaces are non-hierarchial. The main screen for the phone application, for example, contains not one, but three roots.
That sounds like a super obvious, 'wait what the fuck?' sort of observation, yes? As a user, and from what little I know of gui design, this is pretty clearly -not- a good decision. Yet there it is.
How does this work? The phone screen is effectively tabbed, with three tabs to do stuff. The tabs aren't real obvious, but they are there at the top of the screen, and you click on which one you want to enable it.
Coming from a one-root kind of model, I assumed that starting the phone app would throw you into a master screen for doing the most common things, with subscreen branch options. Instead, I'd open the phone app, and get whatever tab happened to be up at the time. First time trying to use the phone:
Open application. What's this call history thing? Obviously a submenu of some sort, I didn't want that, I wanted the phone. Press the back button. Application exits. Start up the application. This call history thing? I didn't want that, I wanted the fucking phone. Back button (to get to the phone.) Application exits. How the fuck do I get to the dialer on this piece of shit phone?
Now that I've properly verbalized and rebuilt my image of how the application actually works, it's a lot easier for me to get around in it, but the way the tabs work is still really unintuitive and clunky.
In fact, I see this in a lot of the phone applications. The back button never goes where I actually wanted it to, and often seems to just leave me stranded either somewhere stupid, or out of the application. Hint: I have a 'home' button which always gets me directly out of the application in one shot. Why would I need a back button which almost invariably does the same thing?