Quote Originally Posted by Mhaeric View Post
Level 10 - Unlocks retainers.
That's level 17.

Another one:
Level 30 - unlocks use of your chocobo as a combat companion (rather than mount)


Quote Originally Posted by Enkidoh View Post
Really, it's not so much level that unlocks things, as more progression in the main storyline/class quests that unlocks content as you go.
Most things are unlocked by quests, but they're not always in the main storyline or class quests. Dye, materia (making, melding, and over-melding), and chocobos (both as mounts and combat companions) are unlocked by sidequests.