Cookies - if a user has them disabled they can't login to quite a few sites until they turn them back on. (Sure session id's can be attached to headers, but that has other security issues) IE 6 & 7- check.
Privacy settings, if locked down as secure as one can make it, this will filter or block 90% (source: thin air) of the web. IE7 & 8 - check. Get the idea, irrelevant stat notwithstanding?
This new "Untrackable" option, if turned on, will prevent you from viewing 90% of the web - eventually. How long will it take developers to add in the same functionality, hacks and patches that make Privacy Settings and disabling Cookies useless?