If you take the list of backwards compatible Xbox games that'll play on the Xbox 360 and then subtract them from every single Xbox game released, you have a list of all your old games that won't work on your Xbox 360. That's the philosophy behind Planet Xbox 360's list of incompatible Xbox games, which gives a good (and slightly depressing) idea of the amount of gaming action you'll be missing out on if you sell your old Xbox.
Whilst the incompatiblity list has some inaccuracies (contrary to the list, Jedi Academy is backwards compatible) it still makes you wonder about the logic behind some of the choices for backwards compatibility. As the article points out, why is an unknown title called Yourself! Fitness compatible, when popular titles like The Chronicles of Riddick, Burnout 3 and Star Wars: Battlefront 2 are missing out on a high definition makeover? Hopefully Microsoft's list of compatible games will be updated to accommodate these and other gaming gems as soon as possible.