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Mobs
Blocks
Items
Environment
Misc
100

This Mob is very common, and used to get food.

Pigs/cows

100

You find this everywhere, even underground.

Dirt

100

Can be crafted from wood

A stick

100

This generates in oceans, and you explore this more than anywhere.

Islands

100

Notch added this in the last update 

New baby animals

200
It floats, and likes to go down.
What is Squid?
200
Red, hot, and is everywhere.
What is Netherrack?
200
Can be cooked, and found in water.
What is Fish?
200
Tool items are found in here more often than on top of the land.
What is Caves?
200

They walk at night and burn in daylight.

A skeleton/zombie?

300
It floats, but doesn't touch water.
What is Ghast?
300
Can't float, but is found underground.
What is Gravel?
300
Colorful, and made of many items.
What is Dyes?
300
Deep, and usually isn't explored much.
What is Oceans?
300

A redstone block crafted with wood

A piston

400
It jumps on you and doesn't die from sunlight.
What is Spider?
400
Can't be touched, can't be mined, but its everwhere.
What is Air?
400
Can't be found in the Overworld, you have to dig deeper than the deepest.
What is Glowstone?
400
Hard to find on land, but can be used to burn things.
What is Lava Springs?
400

You can spawn them, but they wont be implemented officially yet.

A giant

500
It is weird, and creeps up on you.
What is Creeper?
500
Hard, but not in real life.
What is Obsidian?
500

Only obtainable in java god mode

Debug stick

500

Not in vanila but we've all tried making it in vanila 

The ather

500

A minecraft myth

Herobrine