The problem is this, is the person who only has low karma any less skilled in spotting a topic that isn't exactly fit for posting or discussing further, than one who has a lot more karma? Do those who have over 500 karma points, any better at remaining objective when they consider down voting a post?
Introduce reasons and I expect the outcome won't be much different to forums where some people over use a report feature and just select what passes ... generally the acting mods soon realise there's a pattern but it takes time to deal with the BS - actually it's a real time cost.
Presently as far as I see, the system at the moment discourages a user walking into hot topics where the ideologues aren't going to see reason anyhow, so there's little point to pile into the conversation. However if something inappropriate is submitted, a wider field of people can flag it. Sure some people might abuse it, but I would guess there's a list of flagged and dead submissions that staff can look over and review.
Actually for the last week I've been following flagged and dead posts - nothing firm but many are nearly the same or on about the same subject that was posted a couple days before ... if not a few hours before, many are simply advertising their own site, and goodness knows why idiots keep posting here about gummy bears. Today was the first I vouched for a dead post, since it was the shiny flag why not vetting who advertises on a website is very bad ... or hard core porn ads will kill a safe for work or kids will be using it website / service.