I can't say I've had many BAD experiences. At least one that doesn't stand out in my mind. I did however have a funny one in a Trial where most people were new or at least didn't do it enough to remember the mechanics.
It was the Trial with the portals and birds. If you done it...you know which one it is. As the healer for that one I tried to explain the mechanics as we went. People kept getting decimated by the bird mechanic and I was trying hard to keep at least one tank and healer alive. I kept having to prioritize them even though the DPS were dying just as often. I eventually just marked myself and said "Stand where I stand. Half of you will die but it's easier to recover when that happens as opposed to 3/4 or near wipe." After about 35 or so minutes or so we finally clear. I kept at it cause I saw there were a few new people and I wanted to get them a clear.
The best time I had was with a friend I made in game. We played for about 6 months and it was fun to just sit around and do nothing or just run through roulettes. It just kind of sucks they left me in game mail saying they have unsubbed and probably wouldn't be back. I still have them on my friends list in the event they do come back.
I've made other friends since then but none have come close to how fun it was hanging out with them.