Even though the Nether may seem very dangerous, survival is not as difficult as it seems. Here's a list, in order of approximate threat, of the perils you'll find in the Nether:

Fire/Lava

The Nether is a fire marshal's nightmare. It's built of mostly Netherrack, a flammable block that burns forever, and sits on an endless ocean of lava. In addition, lava-falls can be seen frequently streaming from the ceiling, often creating pools of lava or other dangerous obstacles. The worst part is that water turns to steam when placed in the Nether, making fighting fires even tougher. The easiest way to avoid a fiery death is to slow down. Because the terrain in the Nether is comprised almost entirely of the same texture, it's easy to miss dangerous holes and cliffs that may lead to fire and/or lava. Simply watching where you're about to step goes a long way in preventing death. Also remember that you can punch out fires in just one hit. The most important thing to remember is never dig straight up or straight down. Never, never, never. In the Nether, you are your own worst enemy.

Ghasts

You can tell when they're around by the strange sounds they make. If one sees you, if will attempt to shoot fiery explosives directly at you. If you simply want to avoid the Ghast, move so that there is barrier between you and it, and the Ghast will stop shooting. If you want to try to kill the Ghast, you have a few options: You can use a traditional weapon like a bow and arrow or sword (if you're close enough), or you can put the Ghast against itself by punching fireballs with any object to deflect them. A Ghast hit with a fireball will die instantly.

Magma Cubes

Magma Cubes are the Nether equivalent to Slimes. Like Slimes, they bounce around, seek out the player when possible, and split into smaller Magma Cubes when killed. Unlike Slimes, they can spawn anywhere, regardless of chunk or height. Even while in pursuit mode, they are slower than the player, and possible to outrun. However, tiny and small Magma Cubes have very little health (1/2 heart and 2 hearts, respectively), so it's usually easier just to fight them. Huge Magma Cubes have 8 full hearts, so fleeing from them may be a better option if you want to stay safe or have low health/weapons/armor. Due to a glitch Magma Cubes can sometimes jump through blocks, or hide in Soul Sand.

Blazes

Blazes are (probably) the most difficult mob to face in the Nether, and one of the most difficult in all of Minecraft (behind the Ender Dragon and possibly Spider Jockeys). However, they are less of a concern because they only spawn from monster spawners in Nether Dungeons. When you are within range, Blazes will catch on fire shortly, and then shoot a volley of small fireballs at you, setting you and the blocks around you ablaze. In addition, they can spawn in packs of six and have 10 full hearts of health, not to mention the fact that they can fly. Confrontation with blazes is usually voluntarily, since Nether Dungeons and monster spawners are fairly obvious (Nether Dungeons are the only place Nether Brick naturally spawns). If you want to battle a blaze for their valuable blaze rods, you should have plenty of health/food/armor/weapons (Some practice in a save you don't care about can be very beneficial). If you unintentionally encounter one, sprinting away is very effective considering the Blaze's speed. Blazes are harmed by snowballs (3 dmg), so making a snow golem for backup or packing a few stacks of snowballs can be a great help.

Zombie Pigmen

Zombie Pigmen both look and sound creepy, but they are a friendly mob unless provoked. Just leave them be, and they'll pose no threat to you. However, anger just one of them.. and every Zombie Pigmen within 32 blocks will bring their fury upon you. Zombie Pigmen can run just as fast as you can (but they can't sprint), and they deal a fair amount of damage. The is compounded, of course, by the sheer number of Zombie Pigmen that will be after you. If you have sufficient weapons and other supplies, you can try to fight them off, but it is MUCH safer to either:

Hide in a safe place until they become nutral again. Move far enough away so they despawn. Use a portal Set difficulty to Peaceful, then back to your normal setting.

Notes