For some reason, my controller now does next to nothing when viewing a video or listening to a music file after the most recent update. The only buttons that do anything at all are:

L+R+Back+Black = Soft Reset
Right Analog Left/Right = Scroll position (doesn't actually scroll, just moves the indicator bar)

Is there a setting I'm overlooking?

It's because your right thumbstick is not centered (it could be another controller plugged in as well).

Well, thank you very much, that's what it was, a second controller was plugged in. Apparently in the years I've had it, I hadn't had a second one plugged in while using XBMC before. :)