The game need a difficulty curve and tutorial's at the moment both are missing and its showing. ALso the content in normal is so simple and without thought that somtimes when devs decide "this encounter needs to be spicy" a big part of the player base is shocked in for a rude awakeinging. The second trial for example, many people complaint about it. For me this shows the problem, instead of a steadly increasing difficulty, we got "there you go all this fights are idiot proof but hey this one is not". Thats were the problem is rooted. Try and ask a new tank please stop this cast of the boss or a dps please focus adds + dodge mechanics or a simple dispel from the healers. Then i meet player's that have with lvl 90 no clue about: line attack marker, gathering marker, flare marker or line/aoe tank buster marker. Even worse some bosses use "animation" to telegraph their mechanics.

First of all why are these mechanics not in a tutorial, second why is not normal for trash or bosses on normal to teach this on a regular basis. Why are some old dungeons way more difficult then new ones ? Simple, the dev's assume the normal player is a idiot with a crayon addiction. To change this, trash need to be teaching and bosses need a theme that can be recongnized and as the level increases the themed bosses will get more to their array of mechanics up to the max level of the content segment where the boss has its maximum potential. If you use this in a expansion you teach your players step by step the mechanics and how to behave in a fight. At the end is boils down to: dear dev's please stop assume all your player are idiots.