Full list of all 80 Borderlands achievements worth 1,750 gamerscore. It takes around 40-50 hours to unlock all of the achievements in the base game on Xbox 360.
The base game contains 50 achievements worth 1,000 Gamerscore, and there are 5 DLC packs containing 30 achievements worth 750 Gamerscore.
Completed 5 missions in the Arid Badlands
Killed Sledge
Killed Krom
Earned level 5
Earned level 10
Discovered Skag Gully
Discovered Sledge's Safe House
Discovered Headstone Mine
Discovered Krom's Canyon
Won a duel against another player
Rescued a groupmate from death in a co-op game
Completed 15 missions in co-op
Defeated the Rakk Hive, the Vault Boss, Sledge, Krom, or Flynt in a co-op game
Reached proficiency level 10 with any weapon type
Won a duel without taking damage
Killed 25 enemies with corrosive weapons
Killed 25 enemies with explosive weapons
Killed 15 enemies with the Soldier's action skill
Emerged victorious from an arena match
Killed an enemy with the Siren's action skill
Completed all missions in the Arid Badlands
Killed 15 enemies with the Hunter's action skill
Killed an enemy plumber-style
Earned level 50
Killed the Vault Boss
Discovered Eridian Promontory
Discovered Crimson Lance Enclave
Killed Flynt
Killed the Rakk Hive
Discovered Trash Coast
Earned level 40
Earned $1,000,000
Earned level 30
Sold 50 guns to a shop
Completed 5 missions in the Rust Commons
Discovered The Scrapyard
Applied an elemental artifact
Earned level 20
Killed 5 Rakk in under 10 seconds
Killed 25 enemies by ramming them with any vehicle
Mastered the technology of Pandora
Equipped a class mod for your character
Killed 25 enemies with shock weapons
Killed 25 enemies with incendiary weapons
Gamed with fame
Killed 15 enemies with the Berserker's action skill
Complete all missions in the Rust Commons
Race around the Ludicrous Speedway in under 31 seconds
Rescue enough Claptraps to earn 42 inventory slots
Completed the "House of the Ned" mission
Completed the "Braaaaaaaaaaaaains!" mission
Killed Ned. Again.
Killed Ned...sort of
Completed the "Jakobs Fodder" mission
Completed the Prove Yourself mission
Reach the end of the larger challenge in the Hell-Burbia coliseum
Reach the end of the larger challenge in The Angelic Ruins coliseum
Reach the end of the larger challenge in The Gully coliseum
Reach the end of each of the 3 larger challenges with one character
Killed Crawmerax the Invincible
Destroyed the Lance Depot
Rescued Athena
Built the New Car: Monster
Kill each of the loot midgets
Destroy a Lancer while in a Racer
Reach Level 61
Complete all missions in Secret Armory
Pay for a worthless tour of the world's largest bullet
Reached Level 51
Completed Tannis' crazy request
Defeated the Interplanetary Ninja Assassin Claptrap
The Commandant is dead-trap
Killed Ned...again and again and again
Knoxx is at rest...finally
Collected 5 3D glasses
Find all 6 Claptrap Statues
Find 25 oil cans
Collect 15 claptrap bobbleheads
Collect 3 panties, 5 fish in a bag, and 15 pizzas