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.
Killed an enemy with the Siren's action skill
Killed 15 enemies with the Berserker's action skill
Killed 15 enemies with the Hunter's action skill
Earned level 50
Earned level 40
Completed all missions in the Rust Commons
Emerged victorious from an arena match
Killed 25 enemies with corrosive weapons
Killed 25 enemies with incendiary weapons
Killed 25 enemies with shock weapons
Raced around the Ludicrous Speedway in record time
Killed the Vault Boss
Discovered Eridian Promontory
Rescued enough Claptraps to earn 42 inventory slots
Discovered Crimson Lance Enclave
Completed all missions in the Arid Badlands
Earned level 30
Killed Flynt
Completed 15 missions in co-op
Killed the Rakk Hive
Discovered Trash Coast
Defeated the Rakk Hive, the Vault Boss, Sledge, Krom, or Flynt in a co-op game
Killed Krom
Discovered Krom's Canyon
I bet you never thought you'd be here.
Rescued a groupmate from death in a co-op game
Mastered the technology of Pandora
Applied an elemental artifact
Killed 15 enemies with the Soldier's action skill
Reached proficiency level 10 with any weapon type
Earned level 20
Completed 5 missions in the Rust Commons
Discovered The Scrapyard
Sold 50 guns to a shop
Killed 25 enemies with explosive weapons
Equipped a class mod for your character
Won a duel against another player
Won a duel without taking damage
Killed Sledge
Discovered Headstone Mine
Discovered Sledge's Safe House
Killed an enemy plumber-style
Killed 5 Rakk in under 10 seconds
Killed 25 enemies by ramming them with any vehicle
Earned level 10
Earned $1,000,000
Discovered Skag Gully
Gamed with fame
Earned level 5
Completed 5 missions in the Arid Badlands
Completed the "Braaaaaaaaaaaaains!" mission
Killed Ned. Again.
Killed Ned...sort of
Completed the "Jakobs Fodder" mission
Completed the "House of the Ned" mission
Reached the end of each of the 3 larger challenges with one character
Reached the end of the larger challenge in the Hell-Burbia coliseum
Reached the end of the larger challenge in The Angelic Ruins coliseum
Reached the end of the larger challenge in The Gully coliseum
Completed the Prove Yourself mission
Completed all missions in Secret Armory
Killed each of the loot midgets
Rescued Athena
Paid for a worthless tour of the world's largest bullet
Destroyed a Lancer while in a Racer
Built the New Car: Monster
Reached Level 61
Killed Crawmerax the Invincible
Destroyed the Lance Depot
Reached Level 51
Found 25 Claptrap oil cans
Collected 3 panties, 5 fish in a bag, and 15 pizzas
Collected 15 Claptrap bobbleheads
Completed Tannis' crazy request
Collected 5 3D glasses
Defeated the Interplanetary Ninja Assassin Claptrap
The Commandant is dead-trap
Killed Ned...again and again and again
Read all 6 Claptrap statue placards
Knoxx is at rest...finally