I don't think its that simple. On Reddit there is a single karma number. Setting up a subreddit and calling it X lets karma bleed across from X into the main part of the site. In terms of karma transfer X is on topic whatever we call it.
If the off-topic part of the site is to be karma-separated, this will take coding.
Petition: LW should introduce a dedicated off topic area
Why?
1) I want to discuss various topics with people who are both intelligent and rationalist, and i know of no other place where to do it.
2) If find that rationality is getting boring in itself. I need to use it on something.
3) As stated in this comment http://lesswrong.com/lw/btc/how_can_we_get_more_and_better_lw_contrarians/6e3p
the narrow set of topics might actually hurt LW by driving good rationalists away.