The PlayStation 3 has a pretty good interface for playing DVDs, all things told: the key controls are immediately available from the controller, mostly using shoulder buttons as jogs, and I'd guess that most people who have got a PS3 and know what a DVD is have little difficulty using it. The problem arises with the magic.
Magic interfaces are one of those things that are a very good idea, but very difficulty to get exactly right: you want to take the thinking away from the user, so that stuff just works, but this means that if there's any significant mismatch between what you think people will want to happen and what they actually want, your users will start getting frustrated.
This is the area that Apple lives in, at least for the iPod and iPhone: don't think about how this thing works, just assume it will, and get stuck into it. By and large they do pretty well, and by and large Sony do okay with the PS3 also. The key magic that Sony have come up with (or at least the bit that I've noticed) is around what happens when you don't finish watching a film, but need to take the disc out. This is a pretty common requirement with a games machine, so it's not difficult to see why they've decided to make this simple: put the disc back in, and the PS3 will pick up where you left off. This would be an unambiguously good thing, except for a couple of points. Firstly, and less importantly, if you eject the disc at the end of the movie (or TV episode, or whatever), while the credits are running, then the next time you put the disc in it'll jump back to that point, and you have to navigate back to the menu - which some DVDs make much harder than others, because of their desire to show lots of copyright notices for countries I'm not resident in. (I suspect this doesn't happen in the US, and perhaps also not in Japan either.) If DVD authors would stop trying to persuade us that we're criminals, this would be a non-issue.
But the second issue was much more of a pain when I encountered it. Something I didn't notice for ages. Something which is almost never important, because it's simply not something you're likely to want to do. I started watching an episode of something, and noticed there was a commentary by the writer, which I thought might be interesting... and after about five minutes decided it wasn't. So I ducked out of playback back to the menu, and hit the 'play episode' button instead of the 'play with commentary'. And the PS3 helpfully picked up where I'd stopped, with the commentary track.
It took me perhaps ten minutes of turning the machine off and back on, ejecting the disc, and so forth, until I figured out that I could turn the commentary off by resetting the language options on the disc. (For some reason the audio tracks control was disabled for the disc.)
The question, of course, is: is this remotely important? I've been playing DVDs on the PS3 for about eight months, and I haven't run into this problem before now. Most people, I'd guess, don't listen to commentaries anyway; those that do probably only seldom back out once they've started. And most DVDs probably won't cause this problem, because they won't have the audio tracks disabled. So it isn't actually important at all.
The important point is that magic is by its nature opaque; if it weren't, it wouldn't be magic. And, like diesel engines and anything containing a class 4 laser, you can't take apart magic and figure out what isn't working. Instead, you have to build up a conceptual model of how it works inside, and figure out how to game it - which is doubly difficult because the point where the magic needs fixing is the point where the conceptual model that you already have doesn't match the magic in the first place. All your preconceptions go out of the window, and you have to think. Uh-oh.
There are two solutions to this when designing a system. One is not to care: this is the simple route, and has many advantages (including simpler testing). The other is to provide a way of turning the magic off; a kind of less magic switch. Personally I think that the former is a better choice: decide how your system will work, and trust to your own ability to make good decisions. Of course, you may get feedback that suggests you're better off removing the magic entirely, but options force people to think, which goes against the reason you wanted to introduce the magic in the first place.
Just use the best magic you can possibly manage.