The immediate thought is "sites that rely on content being discussable" through things like disqus and the like (mostly because that covers one of my websites). The third party cookies allow for off-site autologins. Of course, allowing them also lets BS like facebook in so I'd love to see browser get built-in automatic third-party blocking with per-site whitelisting of 3rd party domains (I would love to auto-clear disqus on sites I actively use, but not on sites that happen to have it slapped onto a random 1 paragraph "news" item).