What makes XIV's community so unique?

[Lexin]

I would say age definitely is a factor. I can't say for certain, since SE hasn't done a census, but I think it's a safe bet to assume the most common average player age in XIV is somewhere between 20-35. Older players tend to be more patient and tend to be more mature (i.e. they're more likely to mentor a new player, than trashtalk them about how much they suck.) Also with age, people tend to think before they speak, (assuming of course they aren't exhausted or drunk) wheras youger players tend to mouth off.

As for the relative quiet, I'm guessing that most people tend to either use VOIP, or chat within thier own linkshell, I know the one I belong to uses both. New players that don't have one are as you said are invited into LS pretty quick.

Pretty much this. Also if most of the people on XIV came from XI then that could also be the reason for the more mature community. Though I have played some games and even a majority of the people who are 20+ act immature. Seems to be a themepark thing only.

[Elune]

Pretty much this. Also if most of the people on XIV came from XI then that could also be the reason for the more mature community.

I don't know if I'd agree XI had a very "mature" community, but definitly more friendly than most MMO's out there.
I'm on (prolly?) the most populated server atm, so it's not that rare to see shouts in Ul'dah, and the odd convo just popping up. Seems those convos are more about trolling than they are about socializing, though. But my LS' are more than active chatting around (and I'm not shy on contributing either. actually, I think I might even talk too much o_o), so it's not about the people just fearing other internetters.

I think it's more psychological. /shout seems in its essence so rude and intrusive in itself. There's no /help channel to ask for advice or directions. No /general to chat around or idle in. When people see their first LFG/LFM /sh-message, I think they automatically reserve themselves from talking there unless its for just that. Forming/Joining parties. It's become the norm, and people don't want to come off as spammers or nubs in public.

tl;dr
Introduce a /general chat channel for idling, and a /help channel for asking questions. You can always deactivate those channels if you don't want to read them.