There is pretty much no MMO that gets this right. Levelling content, be it grinding mob camps or quest chaining simply doesn't prepare one for endgame content, because the composition for both is radically different. Levelling can be done solo or in a large group, but the latter isn't necessarily reliant on a party balance to make bars fill up. Endgame material is not only almost entirely group-based, but you enter situations that you simply don't see outside of it, such as the poison areas in Aurum Vale or monsters that prepare special AoEs under certain conditions, like the Darkhold Ogre doing Elbow Drop if he's hit in the back by melee.

Players by and large don't know how to immediately react to unknown situations in games, and it's kind of unfair to ask them of perfection on a first run. That being said, there's also a fair amount of people who don't do any kind of research into what the endgame activities are and how to complete them. I was once in an LS run of Aurum Vale where we had to PUG an 8th member who was all too happy to help us. Great DD, but he didn't know how the poison fields worked at all, and didn't speak up that he didn't know about it, either. We were fine about telling him what the deal was, but the truth is that the more proactive players, whether it be through research or disclosure, are typically the better ones.