Yeah, I wonder why Gridania is so popular. It's nothing spectacular IMO. I hope Ishgard will be good looking so it spreads out the population a bit. (: From the map picture it looks like San d'Oria but on a hilltop.
Three words: Location, location, location. A lot of people seem to like Gridania because it's fairly near the center of Aldenard, making it easier for Gridanian adventurers to get to Ishgard. It'd be slightly harder for people from Ul'dah to get to Ishgard, but at least they're on the same landmass; Limsa Lominsa is situated off the coast of an island, so getting from LL to Ishgard might be the new "Windurst to Jueno" or "Windurst to Selbina" trip that nobody wants to take.
That being said, I always loved the Pirate/Sea Horror attacks on the ferry to Selbina, and I had quite a few laughs with friends huffing it from Windurst to Jeuno. I just picked Gridania because it looks so pretty.