Learning how to lead a group effectively will save you hours of waiting around for an instance party. It is generally much faster to scoop up a band of followers than it is to find a leader, and that goes the same for World of Warcraft. Use voice chat frequently, but don't get too bossy - many people don't handle taking direction very well, so it helps to be a bit conciliatory and gentle when giving correction. Getting angry will just escalate a situation and will often end in the dissolution of the group.

The most important thing you can do is to ensure that all of the damage dealers are on the same target. Use the marking feature to call kill orders for trash, and have the same system for every pull. When you get to a new area and you're with people that you haven't adventured before, make sure everyone is on board for the pull, and what's going on with it. Make sure the crowd control folks know what they're doing.

As you can tell, it helps to have some knowledge of the dungeon you're going for before you try to lead effectively. However, it's not strictly necessary so long as you have some skill and previous experience with the game. Generally, tanks, healers or the puller make for the best leaders, as they have an above average level of influence on how the fight will play out. Make sure that everyone understands how a pull is going to go before you do it. Don't get too slow with the pulling, however, as boredom breeds a string of debilitating mistakes.