is there a discussion somewhere on the relative merits of up/down-voting versus upvoting only ?

Yes, it came up here the last time someone made a Discussion post about retributive downvoting. Not to toot my own horn, but I feel I outlined some reasonable issues with that plan in my response.

(Short version: I feel that upvote-only systems encourage cliques and pandering, neither of which align well with LW's culture or goals.)