In my experience it's very responsive and rational. It benefits as a language from being opinionated which is sure to ruffle feathers from time-to-time. Obviously not as big of a community as some others, but that hasn't been an issue for us.
I've seen heated flamewars about important issues rarely, about non-important/emotional issues only very rarely and only in IRC (where the current official policy is "we're a new language so don't kick someone out unless absolutely necessary"). More often with Nim you'll see the occasional "wtf- who implemented this? it needs to change to ---". To be honest, IRC isn't the best place to judge a "community"- trolls and good people venting have disproportionate voice there. And to be honest, if that exchange referenced was disturbing... just hope you never have to be involved in a decision with the Linux kernel team or (the most abrasive example I have first-hand knowledge of) the Apache development team 10 years ago ;-)
Notice there's only one comment there from Araq and it's trying to de-escalate things. If you were to pop onto IRC and ask something technical or philosophical that wanted a rational response Araq or many of the others not represented in that slashdot comment will most likely answer very politely and rationally within a minute or so (I think he's in Germany though? so timing may be an issue).
I think a better "feel" for a community can be gauged by the tone and quality of blog entries coming out and threads in github issues / pull-requests than IRC snippets.
That and the fact that if you decide to use Nim based on its technical merits then you _are_ the community- and at this early stage you can easily influence the tenor of discussions and decision-making for good if you choose.