I would be in favor of a yell feature I think. It would probably get people out of heavily populated cities (Like Ul'dah if it remains the same in ARR as a central hub.) since they can still talk with others clear across the realm. But my only concern is the easy and evident abuse it can bring. While it's easy to chat with others using this method it also opens up completely pointless conversations nobody actually wants to see which could be taken elsewhere. Using Kyri's example I've gotten into TERA recently during the current downtime. On my server's global chat I continue to see conversations about often perverse stuff or discussions that could easily be taken between two people or a private channel.

So trying to keep the channel/linkshell available and moderated could be quite a chore. I think Yoshi-P mentioned already having plans to create some kind of system like this so maybe we'll see it. If not a global then hopefully a regional-type chat channel. Limsa Lominsa would cover all of Vylbrand and the city, Gridania would cover the entire shroud, dungeons, and its city, Ul'dah all parts of Thanalan, its dungeons, Coerthas would have the outlying area and Ishgard, and so on. They did this for Jeuno in FFXI where there was a yell command linking all the parts of the city so we may see something like that again.