Tutorials/Things not to do

Most tutorials are about what to do. This is about what not to do.

Sorted (loosely) by ascending order of obviousness.

Don't Dig Straight Down
This one is fairly straightforward. If you mine the block you're standing on (and don't know what's under you), you're quite likely to fall straight into a cavern (and die/be injured from fall damage), fall straight into a lake (and drown), fall straight into Lava (and burn), or fall into a Dungeon and be mobbed by monsters.

Digging straight down kills. Don't do it.

Don't Dig Straight Up
This isn't quite so obvious. However, if you mine the block ahead of you, you can release all sorts of nasty things directly on top of you. Gravel, Sand, water, lava... it's not fun to suffocate, drown, or burn. Again, don't do it. (Unless you think that torches are a miner's best friend and place a Torch at your feet. Although this protects you from sand and gravel, it is still dangerous because the torch does not help against water, mobs, and lava.)

Don't Try To Kill Creepers On Your First Night
Creepers aren't quite like other monsters. Just read their Minepedia articl- HISSSSSSSSS- BOOM

Just don't try it until you have a Bow.

Don't Mine That Valuable Block
If you don't have a Iron or Diamond pickaxe, many ore blocks (Redstone, Diamond, etc.) won't drop their valuable contents. Instead, they'll just be destroyed.

Wait until you have a decent pickaxe before mining valuable blocks.

Do not play with fire in a forest or wooden house
This is a no brainer. Fire+Wood=??? Arson. Remember kids, Smokey says: "Only you can prevent forest fires."

Don't build your house out of wool
Wool is the weakest building material of all and will be blow up by a creeper with no mercy, which would be bad (go figure). It also catches on fire immediately and will almost always completely burn to the ground, leaving you defenseless. Instead, use dirt for a basic home, it's much cheaper.

Don't use wooden tools
EVER. You only need one wood pickaxe EVER so you don't have to make more wooden tools EVER. EVER.

Don't try to kill mobs in the Nether
Bad. Very bad idea. Unless you're crazy. You have a 90% chance of getting completely blown away by ghasts and a 100% chance of getting murdered by zombie pigmen. Just don't do it.

Don't build TNT traps out of dirt
Basically the same as the house one mentioned earlier, only this time, it's just a pain in the neck to rebuild instead of a disaster that will lead to your death. Try obsidian.

Don't use swords for breaking things
Let's see. Glass breaks at the same rate. Leaves break at the same rate. You have other tools to do things faster, so don't be lazy.

Don't use other tools for damaging mobs
Carry a sword with you, always. Other tools don't do much damage to the mob and they will likely kill you faster than you can kill them. Not to mention the double durability damage.

Don't break snow with bare hands
You won't get snowballs and the snow doesn't come back; case closed.

Don't continually click the powered minecart with coal
I see this time and time again; people continually putting coal in the powered minecart. First of all, it's a waste. Second of all, the minecart is powered for 4 minutes on one piece of coal. You cannot make it go fastr by clicking it faster.