What this mod can do:

• New Weight Restriction and Encumbrance Mechanics
• Reduce all block/item stack sizes
• Terrain affects movement speed
• Stun effect on hit
• Armour weight
• Harder blocks

Those things have changed since the old 1.6 version:

• Added a config (default: true) to disable FoV change. Added to prevent continuously changing FoV due to terrain, weight, stun, etc. slowness.
• Player gets slowed down even in jumps and swimming
• Damage slowness potency has a config (Stun effect. It's a slow)
• Poison effect now drains hunger other than begin able to kill you
• Removed the config to change output torch quantity. Sadly it's pretty hard (and not worth it) to re-implement. You can use simple mods like CraftTweaker.
• Some optimizations
• More features!

Original Mod description:

Now I know adding weight and reducing the items you can carry is not a popular idea. The reason behind it is not a desire for realism, but to encourage the use of minecarts. You go mining and can bring back tons of materials, so currently there is no need for them.

I've also added movement restrictions, for when you walk on certain blocks or when wearing heavy armour. Also getting hit stuns you for a short period.

If you like the survival aspect of minecraft give the mod a try, you might like it.

More informations on the Features and the default configurations

• New weight and encumbrance mechanic
  Made to encourage use of minecarts to move heavier materials. Each block has a weight value, depending on the material; the more you carry, the slower you go.
  Maximum carry weight can be changed in the config. The default value is 768. You can change a lot more settings in the config.
  You can check how much you're encumbered on top right screen.
  
  Example weights by material:
  Rock (stone, ores, etc.) = 1
  Dirt, sand, wood, glass, ice = 1/2
  Cloth = 1/4
  Leaves, vines, plants = 1/16
  All other blocks = 1/32
  All items = 1/64
  
  
• Stack sizes reduced for all items and blocks
  You have to think about what you want to take with you.
  You can set minimum and maximum values in the config.
  
  
• Exaustion when breaking blocks
  Makes the player lose hunger based on the hardness of the blocks broken
  
  
• All blocks are 4x harder
  Encourages cave exploration and discourages staircasing/strip mining
  There is a list in the config that can be used as a whitelist/blacklist. Can be completely disabled in the config.
  You can even set hardness for every single block. By default ores are harder based on the ore.
  
  
• Wrong Tool Punishment
  (disabled by default)
  Breaking a block that needs a tool with a fist will deal damage to the player as block_hardness * multiplier and fail to break the block.
  Breaking a block that needs a tool with an item will not damage the player but will destroy the item and fail to break the block.
  Breaking a block with the wrong tool (diamond ore with stone pickaxe or planks with a shovel) will destroy the tool. If unbreaking is preset then the tool has has chance (based on Unbreaking) to not break, but still the block will not be mined.
  
  
• No Item to Knockback
  (disabled by default)
  If the player attacks an entity without a tool / weapon, the attacked mob will take almost no damage (1/20 of a heart) and no knockback. The no knockback applies even if the attack speed cooldown is below 75% with a tool / weapon, but in this case the damage is applied normally.
  
  
• Poison effect changed
  Poison effect is deadly like wither effect and drains a little bit of hunger, but damages you 3x slowly.
  Made for those who play with health regen off.
  Can be disabled in the config.
  
  
• Walking speed effected by type of block walked on/through
  Walking on grass or dirt will slow by 5%
  Walking on sand or plants (leaves, etc) will slow by 15%
  Walking in snow will slow by 20%
  Walking on Ice will slow by 50% (you know, isn't easy to walk on ice)
  The slow applies for both Player and Entities
  Those percentage slow can be changed or disabled in the config
  
  
• Armor increases Weight
  Defaults at 8 weight per armor point. E.g. With Diamond Armor you'll have 160 more weight
  Can be configured or disabled.
  
  
• Randomised spawning and respawning
  Spawning (upon initial login/word creation) can be randomised
  Respawning (upon death) can be randomised
  Set minimum and maximum block distances in the config
  Example: Have the player respawn 128-256 blocks from where they would normally respawn
  Disabled by default.
  
  
• Sleeping can be disabled and other bed interactions
  Sleeping is disabled by default. You can still set your spawn when trying to sleep.
  Destroy the bed on respawn is disabled by default
  
  
• Stun effect after taking a hit
  When taking damage you'll get slowed down for a short while
  More damage you take, more the effect will last
  The duration is affected by difficulty
  Slowness amount can be changed and can be disabled
  
  
• Less obvious silverfish blocks
  Stone blocks with silverfish inside now act (almost) like normal stone
  Meaning they can broken only with a pickaxe
  Can be disabled
  
  
• New config option to restrict mob drops and change items dropped time until despawn
  Fully configurable for every drop and players drops (on death) will never despawn by default.
  
  
• Hideable Hud
  Everything of the player information can be hidden. Health, Hunger, Armor, Experience bar and Hotbar.
  Health and hunger will hide when over a certail threshold
  Experience bar will always be hidden unless a GUI is open or there are experience orbs nearby
  Hotbar will show when switching items
  Armor will show when taking damage
  Hunger, Health, Armor and XP bar are hidden by default to have more immersion to the game
  Hotbar is disabled
  
  
• Experience amount can be increased or decreased
  Experience dropped from ores, mobs spawned from spawners and global experience can be increased or decreased in the config.
  By default, mobs spawned from spawners drop 25% less experience.
  You can even set if make the experience despawn after more or less time.