I worry that if we decouple "I agree with this" and "I like this", the latter will correlate less well with contributing usefully to LW than the un-decoupled "vote up" currently does.

I don't have any particularly convincing reasons why I should worry that; just a vague feeling that it'll have more tendency to reflect poster status, applause lights, random witticisms, etc., and less tendency to indicate insight and expertise. Then again, looking at my own comment scores, I can't say that my highest-rated comments have been the ones with most insight or expertise behind them as things are...