The biggest problem I have with this issue is, that most 'experienced' players think like this:

doesn't know the dungeon = doesn't know how to play

... and this is so wrong!

There are many very good players out there who know how to play their class and are very skilled, sometimes they are even more skilled as hardcore players, and can teach them a thing or two. But sometimes they just don't progress as fast as other gamers, maybe they started the game not with launch and started playing a month later or they just don't have so much time. So naturally they end up later in the endgame, and will do most of the dungeons there for the first time. But please, don't misinterprete these people as unskilled Players.

Here is a cartoon, that displays it perfectly:

http://www.dorkly.com/comic/49235/a-tale-of-two-gamers