For raid its because people want and expect to play with players with similiar experience and skill.

For dungeons and I dont really understand whats problem to go any dungeon on blind. Since lvl 15 they are all generic always repeating dungeons with just new graphic.

"Hello, first time here" seems to be excuse for low overall performance. Because from my experience people who tend to said that are playing like they play first time not only dungeon but their job also.

I dont really understand what is really need to be explained. Avoid aoes? Kill boss? Kill adds? Repeat.