How about factions with reputation based on completing exam quests (Bayesians? Frequentists? Consequentialists? Bards?), built-in arenas (bets? challenges? predictionbook?), PVP areas where dropping out of arguments without resolving them is punished by karma hits, general chat (permanent open thread?), achievements and meta-achievements ("Karma King: Accumulate 1000 Karma in a Month"?), user profiles (should they show IQ or books read by the players as inventory, as long as they are able to prove that?), random drops ("Downvote Zapper: Negates all downvotes of your next 10 top-level posts and comments."?)

I dunno, LW already seems like it's massively multiplayer enough. Plus its incentive system already tends to lead to spending more time on it, not less.

Very true. It already functions for me kind of like Aleksei_Riikonen describes MMORPGs:

Currently, however, many people such as myself mostly find MMO gaming a threat to their productivity. MMOs can be very fun, druglike even, without providing any utility to valued real-world pursuits such as reducing existential risk and having money to buy food.

Well, it's never kept me from having money for food. And it does increase my long-term productivity when it gives me a greater understanding of the rationalist arts, like in this article and this one. But still, it can be a source of distraction.

Attempting to bend it toward having more benefits to others seems like a prime case where you should watch out for Goodhart's Law. There are many things people can do that can get them quick karma but that they don't currently do because the benefits don't exceed the costs (like turning into a karma-whore). Increase the incentives too much, and the gap between "what gets you karma" and "what's actually helpful" will become noticeable -- and unwelcome.

(Confession: I probably wouldn't have submitted this as a top-level post if I were not reasonably sure it would get a positive rating, and that its rating would be amplified by ten for purposes of karma. That's just insane.)