Another way to think of it: a university needs teachers in the classroom. You can't just do research and have an open admissions policy. Someone has got to be providing training and feedback to the newcomers. Which is work. And you can't just have anyone do it. You need someone who's already had some training. A couple of thoughts:
1) You could feed those vested and proven folks with say, 1000 karma, 20% of their stories with top-level comments in non-descending order:
-- in randomized order instead of rank order, or
-- in inverse order, so they presumably have less cognitive burden to those undervoted great comments. Presumably it is less of a burden to skip over crap than decide if the 59 pt comment is really not as good as the 12 pt comment further down.
2) You could also add a more pre-emptive burden to rep: eg, you can't earn more than 10 points a day unless you vote on 10 new stories first. Feed a daily cookie to them with a popup with the policy, and encourage them to do it.
If you want an experimental focus group to pilot on, feel free to include me.