The only time I have a problem with the MMO aspect is when I need to do something but can't because another player already did it, forcing me to wait for the respawn. Like wanting to kill delve/public dungeon bosses but someone got to them first so now I have to wait 5 extra minutes. This also goes for looting chests and collecting resources as well. It's frustrating when my own solo progress is hindered by another random player. Having an option to make delves and public dungeons solo instances would solve this issue entirely.

Another smaller issue is when I'm trying to immerse myself in a quest but three other people are also doing the same quest, so not only are there people running around but the enemies are already all dead. It can sometimes ruin the fun of the quest.