Shared resources is tricky. Yes, you reduce server storage needed to host 10000 copies of "Your the man now dog," but in turn, you also put more stress on that single file to bear the brunt of thousands of users all trying to access it at once from different YTMNDS.

Similarly to Digg, new websites that enter top viewed often times are the result of having friends vote and view their sh*t. If, however, a site gets 1'd (or buried in the case of digg) enough times after entering the top viewed, it will fall off the front page quicker than a site in top viewed that gets many 5's (or Diggs again in the case of Digg).

After pondering it some more, you could also make YTMNDs like Digg stories: after x amount of days (for Digg its one day, but YTMND is smaller) the YTMND falls off the front page, no matter what. While many times every day there are new "top viewed" YTMNDs, this would prevent sites like "Epic Mount" from staying in top viewed for months (hopefully not).

For example, many ytmnds have viewcounts less than 500 and have been there more than a month (sometimes even longer). Make it so any ytmnd that doesnt meet the criteria of 200 views at the end of one month automatically gets deleted. Then a second criteria that any ytmnd that doesnt meet 1000 views at the end of 6 months gets deleted. This way, a lot of crap that no one watches will get purged.

Deleting all the sites would also be a bad idea. No matter how many you saved, there would always be a favorite that wouldnt be and much crying would ensue. Plus, the minute any large amount of content is deleted people will leave. Kind of like resetting a huge ranking system: people that worked hard over the years now are back to step zero. Instead, perhaps make it so any site that doesnt meet a certain criteria gets automatically deleted.