When running lvl.50 dungeons and someone says that they are new, my typical response is "No problem, I'll explain things as necessary."
Then whenever we get to a boss or there is a particular strategy for a certain pack of enemies that is outside the norm, I will quickly explain what to do even if it is just something as simple as "dodge aoe's and kill adds asap".

However, if you are running a lvl.50 dungeon, I do expect you to know the basics of how to run dungeons and how to play your job and role. For example, if I mark priority targets I expect you to treat them as such since I mark them for a reason, if we are fighting a pack of 4+ enemies use aoe's instead of just single targeting enemies, don't run ahead and facepull things especially if you are new and don't know the layout and location of enemies in the dungeon, and for f**k's sake don't cast regen or med 2 at the start of a pull and if you have the HoT effect still on you click it off before the next pull.
I am always more than happy to work with someone that is new to certain content and teach them, but I will not just carry you through it.

That being said, yes there are jerks and impatient people out there and they can really spoil your fun. Best thing to do is to just not stoop to their level and engage them as that only tends to incite them even more, making the situation escalate and also to try to avoid them by seeking out other friendly and like minded players by using tools like the Party Finder or joining a good Free Company. If you are using the Duty Finder, unfortunately it is a bit unavoidable and you just need to be prepared for anything.