In this RP scenario, you're likely playing with a group of friends, whose goal is likely purely to have fun. They probably don't mind you failing, they probably don't mind you learning as you go. Further, your failure likely has very little, if any, impact on them in the game.
In the WoW LFR/LFD scenario, you're playing with a group of strangers, whose goal is a mixed bag of "just having fun", "progressing my character", and "I just want to be done with this as fast as possible". Your success hinges on the success of the rest of the group, regardless of how good you are (ignoring 5mans for this example because I can solo them at this point). In this situation, is it not rude to assume everyone is playing for the same reasons you are?
This whole problem is partially Blizzards fault. They partner with these 3rd party websites, but then don't have any means of getting their playerbase to them, so that they can improve themselves. They provide the dungeon journal, but make it so thick and technical that only a small percentage of the players can even comprehend it, much less a new or casual player. They provide no information within the game that teaches you how to actually play your class. That's the real culprit behind a lot of the problems expressed in this thread. The other half of the problem being players who just don't give a damn about anyone else. They AFK, auto-attack bosses, don't gem or enchant anything, and expect everything handed to them.