"Subreddits" were discussed here and you can see there some of the pros and cons.
Personally, I would support subreddits. I've been involved in one or two spinoff mailing lists, and it seems a shame that the content of those posts is not available to all on the LW site, if they want to read through them.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.