I, too, have seen some pretty horrible behaviours in poly groups, and have done my best to distance myself from them. It may well be possibl;e for jealousy and envy to be dealt with in a positive learning manner within relationships, but I have seen it running rampant and uncontrolled within a poly community - I feel like saying "but people, don't you realise that it's possible to SHARE?!?!?!"
And yes, I do distinguish this from the cowboy/cowgirl mentality of getting into a poly relationship (or pretending to) with the soul goal of culling the one person away to have a monogamous relationship with them. Sometimes these people feel that poly is inherently wrong, are attracted to the person, and feel that the reason someone is poly is that they haven't met the right person yet, and, of course, they assume that they are that right person. I feel that that is pretty disrespectful of differences in lifestyle, but that's just me.