Well, there really aren't that many options for a button that would change the way we play first person shooters forever. Most of the ideas have been done before in some way, shape, or form.
Alright, I just read the article that revealed what the 'X' button does and that's not at all innovative. Apparently the Halo developers haven't played other fps games before. Or maybe they're pretending like the others don't exist. XD