Here is the full list of all 50 Marvel's Avengers achievements. It takes around 100-120 hours to unlock all of the achievements on Windows.
Complete the "A Global Offensive" mission chain
Locate the Ant Hill
Destroy the AIM arc reactor
Fully upgrade any gear item
Complete 10 assignments
Open 100 strongboxes
Recruit Tony Stark
Collect 500 gear items
Break into 30 Depots
Earn a five star rating on an objective without any team member taking damage
Purchase an item from two different faction vendors
Reach Hero Level 5 with five different Heroes
Earn a 100% rating on a mission without any team member being downed
Complete any mission in under three minutes
Defeat any 20 different enemy types
Recover the SHIELD Protocols
Collect three Artifacts
Defeat 50 enemies with Assault Heroic or Ultimate Heroic abilities
Reach a combo of 20 hits or higher
Recruit Bruce Banner
Discover the location of the Chimera
Escape the clutches of AIM in New Jersey
Defeat 10 or more enemies with a single Heroic ability activation
Reach the VIP balcony
Collect a full comic set
Reach Power Level 150 with any Hero
Earn a total of 250 Hero Levels across the entire Hero Roster
Reach rank 25 in any faction
Reach Hero Level 50 and purchase all skills for any Hero
Complete 15 Villain Sectors
Complete 100 assignments
Open 50 Caches strongboxes
Complete five Hive missions
Complete the "Iconic Avengers" mission chain
Complete 50 Hive missions
Complete 30 War Zones at Challenge III or higher rating
Earn a 100% rating for any War Zone at Challenge IV rating
Complete HARM Challenges I to V
Collect 50 intelligence files
Rescue 100 AIM prisoners
Equip all Legendary or better gear with any hero
Fully upgrade a major artifact
Complete an Elite Heroic Hive
Recruit Black Widow
Rescue Inhumans from the research facility
Stop the Helicarrier from crashing into New York City
Reach the AIM space station
Rescue Captain America
Reunite the Avengers
Defeat MODOK and the Kree