A successful raid usually consists of 10 or 25 people working together, under the direction of the raid leader. It does not consist of the raid leader plus 9 or 24 morons that the leader berates, scolds, ridicules, and basically drags along for *his* boss kill. A decent raid leader remembers that there are actual people behind all the toons in the raid. Screaming put downs is not going to accomplish anything.

Instead of telling someone how badly they suck in front of the entire raid, why not take the chance to talk to them after, and explain the problem. Like "You seem to be having a hard time moving out of gross stuff on the ground, is there anything you think we can do to help the situation?" It may be something as simple as making an adjustment in the video settings, to be able to see the ground effects better. Screaming "you suck" at them won't get that fixed, and odds are you will lose what could be one of your top raiders.

TL; DR:
A good leader solves problems and accepts he isn't perfect either. A bad leader screams "you suck" and blames everyone else except himself.