Limited number of buttons doesn't limit that much. 4 directionals and a dial are pretty much all you need, but how they are used to interface with the screen is where problems occur. Baring the entire argument by the 'I know how to get all of the functionality, so don't bore me with extra menus'-camp, hidden function makes an interface hard to use. As any course in human-computer-interaction can tell you, not indicating what a button does should be avoided..
Heck, even though I know that the Down button brings up the menu, I often press pause instead, simply because the menu is at the top of the screen, and pause is at the top of the player. Perfect example of how mapping is something the mind does naturally and therefore should be taken into consideration. And yes, I know all of the reasons why it should be down, but that doesn't mean it works for me..
Granted, this is a device that will always have to break some UI rules to provide the functionality it does, simply because of the limitations of the interface, but it doesn't mean breaking them when it can be avoided..