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