• Works just fine with anywhere from 1-4 players
• You can have a mix of friends and random players if you want a full team but don't have a group of four to play with
• The game is strictly cooperative so there's no incentive to screw each other over, fostering a generally good multiplayer culture
• There's lots of stuff to unlock but none of it is necessary for a fun and functional build
• Teamwork is encouraged and effective
• Missions are randomly generated but you've got years of additions to the random pools for lots of variety
• Absolutely no pay to win stuff
• There's cosmetic DLC, but there are still absolutely loads of great cosmetic options in the base game

Edit: also, rock and stone