A few factors. First, Natal has to be huge, like Wii Fit huge. AND this gen has to last until 2013 or later. If only the first factor, the next Xbox will cannibalise the sale because it will obviously support Natal. If only the second they won't be able to pull off the numbers each year. IT doesn't seem likely to me, but it probably does to an MS fan.
"Now, a fun game should always be easy to understand - you should be able to take one look at it and know what you have to do straight away. It should be so well constructed that you can tell at a glance what your goal is and, even if you don’t succeed, you’ll blame yourself rather than the game. Moreover, the people standing around watching the game have also got to be able to enjoy it." - Shiggy
