The Elements:

The firebenders are nice and balanced, they have decent enough sub-categories, and good strengths/weaknesses.

Normal Firebending
Weak against: Water
Strong against: Metal/Plant

Bending Sub-Categories:
Lava/Magma
Weak against: Water
Strong against: Metal

Lightning/Electricity
Weak Against: Earth
Strong Against: Water, Crystal

AIR

Airbenders got screwed: they only have one type of bending, with no sub-categories.

Airbending:
Weak against: Earth
Strong against: Plant

Earth

The Earthbenders are lucky: their weaknesses mostly involve other Earthbending techniques, and they have cool sub-categories.

Normal Earthbending:
Weak Against: Metal, Crystal
Strong Against: Fire, Thunder/Lightning

Bending Sub-Categories:

Metalbending:
Weak against: Lava/Magma
Strong against: Earth

Crystal:
Weak against: Earth, Thunder/Lightning
Strong against: Metal

Sand:
Weak against: Air, Fire, Water
Strong against: Chi

Water

The Water benders not only get nice strengths against other elements, but nice subcategories.

Normal Waterbending:
Weak against: Thunder/Lightning, Plant
Strong against: Fire, Lava/Magma

Ice:
Weak against: Lightning/Thunder
Strong against: Plant

Bloodbending:
Weak against: Earth, Fire
Strong against: Chi

Plant:
Weakness: Fire, Ice, Wind/Air
Strong against: Water

Chi

The element of Chi is a specific element, used by non-benders. It can be used to disable their bending enemies (like Tai Lee,) or it can be channeled into powerful physical attacks. (This gives people who are not benders higher strength than those who are.)

Chi Blockers are specially trained non-benders, who can stop a person's bending for a certain time. While only the Avatar can remove bending for good (NOT allowed here,) blocking Chi can be very dangerous for not only benders, but non-benders as well.

To Train as a Chi Blocker:

You must have over 200 posts.
You must have proven a capable RPer.
You will have to be taught by another Chi Blocker.

Tai Lee, Amon, and Tyrral are the ONLY Cannon Chi blockers, and are the only people exempt from the rule of learning how to block Chi.

There are FOUR stages to Chi Blocking, and each one has a specific post count:

Beginner: 200 posts. Chi blocking skills block enemy Chi for 5 posts.
Intermediate: 500 posts. Chi blocking skills block enemy Chi for 10 posts.
Apprentice: 1000 posts. Chi blocking skills block enemy Chi for 15 posts.
Mastery: 4000 posts. Chi blocking skills block enemy Chi for 1 full page of posts.

At each stage, you must take an exam with a member of staff in order to validate your ability to RP fairly with these abilities.