The game is too easy? No problem. This mod adds several new difficulty levels to Minecraft. After peaceful, easy, normal hard, there is now (in order) expert, nightmare, fiendish, cataclysmic, pandemonium, unutterable and insurmountable. I do not recommend going straight to insurmountable difficulty. I picked these names for a reason. The old pandemonium difficulty from version 0.1 is a cakewalk compared to insurmountable. And that is already really really hard.

Here is what happens:

Expert:
-Mob damage factor 1.5
-Arrows have a 1% chance to poison the player
-Mobs have a 20% larger following range
-Skeletons have a 60% larger following range
-Zombies have a 10% chance to spawn with an iron shovel or sword
-Skeleton arrows are more accurate and fly 50% faster

Nightmare:
-Mob damage factor 2
-Arrows have a 2% chance to poison the player
-Mobs have a 40% larger following range
-Skeletons have a 120% larger following range
-Zombies have a 15% chance to spawn with an iron shovel/sword or a diamond sword
-Mobs do not despawn that easily
-Mobs are 5% faster
-Spiders can trap players in cobwebs if they are close

Fiendish:
-Mob damage factor 3.5
-Arrows have a 5% chance to poison the player
-Mobs have a 60% larger following range
-Skeletons have a 180% larger following range
-Zombies have a 20% chance to spawn with a weapon (including diamond axe now)
-Mobs are 10% faster
-Torches cannot be placed
-Placing lava might spawn a blaze
-Zombies break blocks
-Creepers spawn charged

Cataclysmic:
-Mob damage factor 5
-Arrows have a 10% chance to poison the player
-Mobs have a 80% larger following range
-Skeletons have a 240% larger following range
-Zombies have a 25% chance to spawn with a weapon
-Mobs are 15% faster
-Creepers can launch TNT at the player
-nights are longer
-the equipment of zombies and skeletons can have strong enchantments
-zombie pigmen are always pissed

Pandemonium:
-Mob damage factor 8
-Arrows have a 20% chance to poison the player
-Mobs have a 100% larger following range
-Skeletons have a 300% larger following range
-Zombies have a 30% chance to spawn with a weapon
-Mobs are 20% faster
-Creepers have an explosion strength of 8
-short days, nights stay forever until the player sleeps
-zombies can build
-3% chance for an enderman in the end or a zombie pigman in the nether to be replaced by a stray
-mobs sense players through walls in a range of 16 blocks

Unutterable:
-Mob damage factor 12
-Arrows have a 50% chance to poison the player
-Mobs have a 120% larger following range
-Skeletons have a 400% larger following range
-Zombies have a 45% chance to spawn with a weapon
-Mobs are 25% faster
-short day followed by eternal night
-zombies have a 12.5% chance of spawning more zombies when damaged by a player.
-sleeping is not possible
-zombies have a 5% chance to spawn with a lava bucket
-5% chance for an enderman in the end or a zombie pigman in the nether to be replaced by a stray
-mobs sense players through walls in a range of 32 blocks

Insurmountable:
-Mob damage factor 16
-Arrows have a 100% chance to poison the player
-Mobs have a 150% larger following range
-Skeletons have a 500% larger following range
-Zombies have a 60% chance to spawn with a weapon
-Mobs are 30% faster
-instant and eternal night
-zombies have a 25% chance of spawning more zombies when damaged by a player
-zombies have a 15% chance to spawn with a lava bucket
-1ß% chance for an enderman in the end or a zombie pigman in the nether to be replaced by a stray
-mobs sense players through walls in a range of 128 blocks

I have heard of a bunch of awesome ideas to add. One thing i really want to improve are zombie hordes. If you have some good ideas which buffs Zombies should get if they are in large groups, let me know.