In Final Fantasy XIV servers are divided in Data Centers (Regions) and then worlds. To make sure that the population of each worlds is balanced the team implemented a system of "Congested Worlds" on which you can't create new characters.
If you're having trouble creating a character on a server it might be because it is registered as a congested world.
Here is the current status of worlds for the North American Data Center updated for Patch 5.35:
Aether Data Center
- Adamantoise
- Cactuar
- Faerie
- Gilgamesh
- Jenova
- Midgarsormr
- Sargatanas
- Siren
Primal Data Center
- Behemoth
- Excalibur
- Exodus
- Farmfrit
- Hyperion
- Lamia
- Leviathan
- Ultros
Crystal Data Center
- Balmung
- Brynhildr
- Coeurl — Preferred
- Diabolos — Preferred
- Goblin
- Malboro
- Malteus
- Zalera
As you can see, with Patch 5.35 of FFXIV all the Congested worlds are back to normal, with new preferred worlds on the Crystal Data Center.
If you have any questions on the World Classification or want us to report on the other data centers, please feel free to let us know in the comment section below.