I am hard of hearing and have played 3 beta's. I also play MMO's ever since Meridian '59 so i have seen the genre evolve in all those years.
Where in the beginning everything went over text and people were all adapted to that, communication nowadays make more use of VOIP than text.
So as an hairing impaired it is not just missing out on a great deal of social chat but also i am missing out more and more of essential game play. In big scale battles, PvP but also dungeons etc. there is simply no time to type out orders and for me its hard to keep up with all the chat spam to pick up what is going on. People will be on VOIP during those battles and once the leader said, in the heat of it all, go for target X, there wont be a lot of people realising that i didnt get that order.
If the game had a simple way to mark target X it will be obvious for me that thats the target.
Tactics nowaday will mostly been shown on Youtube and all i can do there is watch it but not hear the actual explanation of it. I know that this isnt a fault of the game, but it does show how much more i will need ingame visuals than the avarage player.
Also in pve a lot of hints and hints to new quests are given by overhearing npc gossip. I cant hear them. In beta i used Wykkys addon which gave me subtitles and that worked fine for me. I hope I can still use it next week, but I fear the worst.
And no, npc gossip can't be turned on in the chatbox with the normal settings, only the chat of quest npc's can.