Communication - the biggest gap to progression
There is a sad lack of social interaction, and particularly the need for social interaction in this game... until the very end game.
You can very easily do the expert dungeons with literally 0 communication. Even Titan HM, the first real progression road block requires very little/no communication. Basically, the only thing you need to communicate for is to find a regular group (which is bascially mandatory if you want to do the later turns and X modes).
MMOs of the past have emphasized the need for communication, even for casual players. This game seems to have completely removed it... until you really need it.