I don't have a problem with cross-origin sharing of things like images, and I believe blocking these would break a lot of web pages.
However, I would quite like cross-origin blocking of things like flash and scripts. After all, that flash program that kicked the whole thing off probably wasn't loaded from the host web site. That seems much more sensible and low-impact to me. It also has a side-benefit of forcing ad networks to fall back to static images, which has to be a good thing.
Most ads are served in iframes, which get their own origin independent of the parent window. Unless you want to block cross origin frames, then this is still a problem.
However, I would quite like cross-origin blocking of things like flash and scripts. After all, that flash program that kicked the whole thing off probably wasn't loaded from the host web site. That seems much more sensible and low-impact to me. It also has a side-benefit of forcing ad networks to fall back to static images, which has to be a good thing.