Hazards

Hazards are common dangers or things that can cause injury/death.

Danger levels are comparative - the lowest is one.

Physical Hazards
Mobs

Found at all heights from the base of the world, although most common at water level.

Danger varies depending on mob type, see Zombie, Skeleton, Spider and Creeper.

Best way to counter the danger is to use walls to block their movement (and attacks in the case of skeletons), use armor to absorb some/most damage, be well-armed, and survey surroundings carefully.

Danger level: 3 (although greatly varies on time and location)

Note: Forest areas are both good and bad in respects that 'charging' enemies (all but skeleton usually) may get stuck between trees/leaves, but the player may also get pinned in areas while attacked.

Water

Found at all altitudes but in greatest quantities around the main landmass.

Danger is from drowning - any time there are two water blocks in depth there is the risk of drowning.

Best way to counter the danger is to move around the water rather than through it, to keep jumping which makes the player swim on the surface or to build a bridge through the water. Ponds may be drained with buckets.

Danger level: 1

Lava

Found usually below water level, further below water level exponentially increases the chance of finding lava pockets.

Danger is from contact - players are set on fire and require water to extinguish themselves. Lava pockets are often difficult to detect before being tunneled into, although as long as you do not mine directly up or down you will usually be able to outrun the lava flow.

Danger level: 3 (due to the rapid and often unstoppable damage if contacted)

Note: Using water the lava can be transformed into harmless obsidian - players should be wary of digging through or around the obsidian in case the lava is more than one cell deep.

Gravel/Sand

Found at all altitudes - sand anywhere from about 5 cells below water to varying heights above, gravel can be found at all altitudes.

Danger is from gravity - these blocks are affected unlike most, and undermining them can cause suffocation if trapped.

Danger level: 1 (players can usually just move to one side or another to escape)

TNT

Only man-made, although if placed/detonated incorrectly is incredibly dangerous.

Danger comes from both the explosion and the resulting crater, which can deposit players in deep caves, causing fall damage.

Danger level: 1 (if placed correctly); 4 (if placed wrongly), 1 (fall danger)

Other Hazards
Dungeons can be particularly dangerous - bursting into one by accident with TNT can spell not only a single death but also a mob-ridden section of your base. Most people consider dungeons worth raiding however, because if lucky you can get stocks of redstone dust and gunpowder, saddles which are uncraftable, and music discs, which are otherwise very difficult to obtain.