Progression through Eorzea

In the old 1.x version of the game, there was no "order of zone progression" as in other MMOs like World of Warcraft or The Old Republic; each starting region has levequests, quests, and monster camps appropriate for players of all levels. There was content appropriate for levels 1-50 in each of the three nations, so it was entirely possible to level all the way to cap without needing to leave one's home region. It wasn't uncommon for various activties (storyline missions, faction levequests, parties, etc) to take a player to Coerthas and Mor Dhona, but strictly in terms of leveling, it wasn't necessary at all.

As this was a particular complaint among many players, all of the zones have been completely redone in A Realm Reborn. All of the regions are broken into smaller unique, more traditional "zones", and provide an intended order of progression as you move through the main story content. Figuring out this order isn't too complicated, as the main storyline and your class's hunting log will both guide you to the proper zones where you can find content appropriate for your level. Here's a rough list:

1-15: Depends on your starting city. Limsa Lominsa: Middle La Noscea, Lower La Noscea, Upper La Noscea; Ul'dah: Central Thanalan, Western Thanalan, Eastern Thanalan; Gridania: Central Shroud, North Shroud, East Shroud

15-20: Dungeons (Sastasha, Tam-Tara Deepcroft, Copperbell Mines); Eastern Thanalan, East Shroud, Upper La Noscea

20-25: South Shroud, East Shroud, Upper La Noscea

25-30: Southern Thanalan, North Shroud

30-35: Eastern La Noscea, Southern Thanalan, South Shroud, Central Shroud

35-40: Coerthas Central Highlands, Eastern La Noscea, Upper La Noscea

40-45: Coerthas Central Highlands, Mor Dhona, Northern Thanalan, Upper La Noscea

45-50: Mor Dhona, Northern Thanalan, Outer La Noscea