When you're driving a car, the worst thing you can do is trust the other drivers on the road. It's a sure-fire way to get into an accident. And when you're levelling through quests in vanilla, the worst thing you can do is trust other players. Many levellers are just learning their class and WILL screw up, and it's just a fact that the screwups of your party mates get your toon killed 10 times more often than your own screwups do. So I'm not saying don't group up, because you HAVE to group up to complete some quests, but silently and politely assume that everyone in your group is an idiot who doesn't know what they're doing, and do what you can to maximize your own chance of survival.
This is especially important if in real life you are a military person. Other players aren't your highly trained deployment unit buddies. They're a variety of civilians who have no conception of the fluidity of battle, the need to plan extensively and adjust to how things change, the extreme importance of not acting like they're ADD when going into a dangerous situation, and above all the essence of being slow and deliberate and well-co-ordinated in starting a fight.
I'm telling this to newbs from years of bitter experience with having PUG quest groups get my toon killed. Don't let it scare you away from grouping up, but be smart enough to be paranoid when in a PUG.
