High karma on hacker news may not translate to a high reputation on a writer's blog, but it would pretty much guarantee a civil, thoughtful comment. In this way karma is pretty portable, you just have to give everyone karma somehow. Maybe start off by trusting friends of people with high karma.
That may be what you look for on HN, but that's not true of all forums (I trust that you can think of one obvious counterexample).
Even look at Reddit - trolls are usually downvoted, sure, but a lot of people with high reputation are really just novelty accounts. That's not bad for Reddit, but that's not what I would associate with high reputation on HN.
And finally, look at one of the comments below about gaming the system - that's the problem with having a (displayed) numeric value associated with one's reputation.
