I had the same issue with 4-man dungeon not to long ago. I was holding aggro but the healer insulted me and just went on about how I wasn't marking and looking through his characters he hasn't ever tanked before. none of the dps were complaining and I don't mark normally, but out of the countless dungeons I have run all the dps and healers just tell me to rush through it and hold aggro. just this once I got told to mark. So it's pretty interesting imho.