My take on light & friendly versus straightforward -- it is always, in my opinion, possible to be straightforward while also avoiding being a jerk. Anne Lamott, in Bird by Bird, says "point with the sword of truth, don't slash with it" (paraphrased). That's my feeling.
Now, where the line is where people go from feeling "this is constructive" to "my lord, you're a dick" does vary. When I teach, I try to warn people as stongly against the danger of being wishy washy and refusing to say anything negative as about the danger of being a jerk.
In the two years I tuaght creative writing on the college level, I came across only two instances of critiques that required intervention. (1) When one student informed another that "The author of this story should be on antipsychotics," and (2) when one student informed another "You're not getting any better; why are you even still writing?" (One of these was intended to be a joke, and the situation was easily resolved. The other one was not a joke, and required a lot of reorganizing of the class to cope with.)
I *have* been in other critique situations as a participant where someone got nasty, and it's sad to say, but one really nasty person can poison a group which is otherwise excellent.
However, being nasty does not equal being straightforward. Straightforward, negative critiques can be totally awesome. Things only get problematic when one person stops treating the other person as a human being. People in a crit group are friends, peers, and to some extent business contacts. It's useful to treat them as professionals. And as long as that gets done, things usually go swimmingly.
So, yeah, in sum, I don't think it's worth it for anyone to worry about not joining the group because you're worried you'll be a dick. Unless, you know, you actually are trying to hurt people to make yourself look better. In which case, one of my Clarion West classmates Heather Lindsley ("Mayfly," "Just Do It") has a solution... every time you think of something really clever and devastating to say, give yourself one cool point for how clever you are, and another cool point for your restraint in staying silent. ;-)