Unfortunately, talented though the dev's at Blizzard are they have yet to find a way to change human nature. Adding a rating system would only hurt good players in the end, not the bad ones. At least if the popular perception that there are more bad players than good ones holds true.

LFG is just like going to the airport, or a bus station. You see the best and worse dredges of society thrown together. Add long wait times, delays, and bad vending machine food with gross bathrooms to the mix and it's a volatile cocktail. Only in blizzard's case it's people who stand in stuff, forgot to repair before they enter the instance, long que times for dps, and people with language so foul you wonder if they even graduated from middle school.

Rating systems are a bad idea, they'd just be abused. The only true fix is for people to relax their expectations a bit, be a little nicer to everyone they meet, and accept the bad runs with the good ones. The LFG concept is more or less the same as it has always been, blizzard simply made it easier to use. I use to spend 2 hours spaming trade for a tank, or a dps with the proper CC for a group in BC. Even after all that work, the group could still fall apart before the first boss if the guys in it were tools.

People were just better about grinning and baring it in those days, cause they had no other options.

Blizzard does their best to police people on their forums and via GM ticket when there is harassment on the same realm. Even that taxes their resource.