Mechanically, I struggle the most with pet management in most games. I tend to avoid pet classes because I'm just not good at that. Tanking, healing, other forms of DPS, no problem. Pets? I suck.
Conceptually, I struggle the most with grinds. Doesn't matter whether it's a faction grind or a skill grind or an exp grind. If I have to repeat the same action over and over and over just to advance a number, in order to achieve a goal, I will really get bogged down by it. However at the same time, I am the person who just goes out and does things and then ends up tending to outlevel quests and areas before I'm finished with them - so I'm also the person who will turn off XP gain if I'm able to do so.
In-character, I struggle the most with inventory management. Like, I know I have those things in a bag here somewhere.....
Socially, I struggle with people who are very achievement-oriented. I'm very much an experiential, journey > destination type player, and so when I meet someone who is focused the opposite way, it makes it hard for me to relate to them at all.
I realize I took the question in a few different directions but, I dunno, maybe it's interesting to someone.
The struggle is rarely with the game, and mainly with the players. Obviously not all of them. Just the arrogant ones who try telling me how to play my class or the impatient, egotistical ones trying to burn through content all the while constantly pushing people to hurry up because they have somewhere else to be outside the game.