While it may come in handy for lists such as "TV Pets" which would fall under the category of both television and animals, searching by "keyword tag" should help a little when it comes to such lists.

I don't think it's needed. As it is, there's quite a mass in each category. I do think we need a few more categories. Some of the lists seem a bit misplaced.

Has potential for becoming unwieldy. If a more uniform standard for list naming and categorizing were established this "new" feature would be unnecessary.

Technically, this is way more complicated than you would imagine AND it would really open the door to people spamming their list into all categories, even irrelevant ones.

The primary purpose of the Weblist tags is to enable exactly this sort of functionality - the ability to cross categorize your lists, and the ability to find related lists across categories.

Another non-essential improvement, but worth consideration, adding the ability to add lists to multiple categories could increase the ability to find an existing list and decrease the chance of list duplication. For instance, I created a weblist of television theme songs (rather popular recently), which could easily fit under both music and television. On the other hand, the number of categories assigned should be limited to prevent abuse.