Federico posted about some work he was doing on making Firefox not cache as many uncompressed bitmaps in memory. I was playing around with the cache stuff and noticed something: my Firefox cache is full of youtube videos. YouTube videos aren't exactly the best thing for Firefox to cache. My internet connection is fast enough that streaming the videos works just fine. I suspect that most people who use online video frequently do so on a connection that can support streaming (otherwise, YouTube would be painfully slow, and they'd go do something else).
Even worse is if you listen to a flash-based media player. The MP3s that this downloads are cached just like anything else. So if you listen to 50 MB worth of music your disk cache gets blown away.
Probably LRU isn't the best technique to use here. I'm not sure how one would evaluate various choices (what is a representative test set of browsing sessions?)