This has been discussed before.
I don't think you understand why I don't like the idea of LW becoming a forum. LessWrong is less a collection of cool people than it is a particular niche in online discussion. By changing that niche, you change the demographics, and probably increase the raw number of participants. Besides this reducing the signal to noise ratio, you are relying on the core seed group to step up their gardening, something I see very little evidence might actually happen. You also implicitly rely on them not changing their standards and expectations. People generally behave differently in different on-line venues.
Even putting these concerns aside, Karma systems have very different dynamics with different scales and the effects are nonlinear.
Something that might make this less of a problem, if it does get implemented, is having two separate karma systems for the two areas. This way, standards for main/discussion sections can be maintained, while we can have the alternate discussions elsewhere (with a karma system of its own).
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.