I happen to be a pretty good leader, I've done every major quest in the game at least twice, solved most of them myself, and gotten through some tough situations. I was on the quest right before Denravi (where you have to align the portals)...
oh btw if anyone reads this and has a foolproof strategy, please tell me. This is the toughest quest I have encountered in GW and I have not found a strategy at all. There are so many White Mantle, and they run so fast, and just keep coming, theres no real strategy. Usually if you stop and fight they either take all the portals or you get CREAMED.
Anyway's, the point is, someone in my group had sprint, and my strategy was to have two people waiting at the crystal, have one person sprint over to one holder, have the next person sprint to the other and win the quest. I had been asking the other warrior in the group forever for stuff, backup, questions about what weapon they had etc. Never an answer. We got to the part where you have to align the last gate and run in and do the final part, and when we were in the middle of planning, they just opened the gate and ran in, didn't even let us prepare. We all died. Afterwards, when we all spawn at the outpost, they go "Oh sorry I thought people were ready". This was after 15-20 minutes of SILENCE. I have run into this at least 5 times in the game so far. Now I know some people may not speak English as this is an international game, but every single one of these people showed in some way that they could understand what we were saying. Has anyone else run into this? It makes me want to just run around with henchman, or only group with people that I know (which isn't always easy to do). The funny thing is, I had one group for the quest where you have to kill all the Charr and let Rurik out of the gate, and he didn't utter a word the whole mission, but followed what I said. 2 of our group members died, and it was just he and I left, and we ended up killing like 20 Charr alone without him uttering a word. Afterwards he said "Sorry I've been playing for 20 hours and didn't feel like typing." Still a damn good player

The other thing... and I have no clue how to fix this except have a great guild/group of friends and go out and play, is that people do not listen. There are so many times where people run ahead into situations and screw things over. I had a monk the other day just run onto the platform where you fight Hablion and engage him. He got 5/6 group members killed and we almost had to redo the quest. I know I'm going to run into this in any game, but Guildwars it's just essential to take it slow and listen to people.
I wanted to see how others have dealt with this and what they have done to solve it (especially since you can't kick people out of a quest once they are in).