From what I would consider least to most important:
The gameplay itself:
It’s just a lot of fun. Whether I’m going commando with Roland, riding meat bicycles with Krieg or annoying the ■■■■ out of my friends
in real life with Claptrap. There’s lots of little mechanics that eventually build up to give each character a unique playstyle and each weapon a specific use, many unorthodox combinations that are such a joy to make work, the combat flows nicely and the promise of treasure somewhere, sometime, always keeps you on the lookout to spice things up with a shiny new toy. It discourages complacency and rewards adaptation, which I value greatly in every game since I’m naturally a very complacent person.
The characters and writing:
I don’t know what exactly makes it so, but I’ve always had a soft spot for some characters in the games. It’s just nice to have a shooter where NPCs are not categorised into “gruff shooty dude” “naive young shooty dude” and “obligatory girl”, maybe with “brainy guy that gets made fun of”, where every semblance of personality is relayed through bad one-liners. Not that we don’t have that here (I’m looking at you, BL2 Roland…), but it’s a huge step. Even some minor NPCs you hear for one mission show a lot of potential.
Special mention has got to go to Krieg, my forever favorite VH whom I relate to probably a little bit too much and it breaks my heart every time I watch A Meat Bicycle Made for Two or hear him scream “I CAN’T TAKE IT!” when activating Release the Beast, all the time I’m running and gunning I’m reminded that the man on the inside just wants to get out, to get away, and this kind of execution is a much better spin on a mentally troubled protag then “generic whiny teenage/young adult disillusioned with life for no good reason”. Krieg’s seen ■■■■, he’s done even worse but he’s not brooding - he’s so completely off the edge he just keeps going forward doing his best to survive while fighting himself from the inside, hoping things will get better for him even though he himself likely never will.
Smaller special mentions: Patricia Tannis, for similar reasons to Krieg, but she hates music so I will never love her as much (who the fuck hates music?), and because the audio logs in BL1 really contributed to that game’s atmosphere; Brick, for being the manliest man to ever cry over the loss of his puppy and general awesomeness; TK Baha, for being the first NPC in my gaming experience whose death really upset me, the poor, jolly old man; Handsome Jack, an amazing villain, 'nuff said but please just let him die already, m’kay? 3 games are too much; Claptrap, for being an unexpectedly tragic character that still makes me laugh despite his (or because of? I’m a bad person) shortcomings; Zarpedon for actually getting my respect to the point I didn’t really want to kill her, and also the one speech in Don’t Shoot the Messenger that made me tear up a bit; probably a lot more that I’m forgetting, but I’ve already spent a lot of time on this list
The atmosphere of BL1
I could rant a lot, but I think I’ll just plug an old post of mine that explains everything in true broody teenager fashion
Tl;dr the art design combined with gameplay and writing creates an amazing story from the very second you hear the guitar in the Arid Badlands background music. The game just oozes climate, and it will always be my place to go not just when I want to play an amazing game, but when I need some emotional space, I know the wasteland will be there for me to admire and contemplate; a feeling irreplicable by any game, including other BL games. The creative team had a concept for a game that was their own, unhindered by expectations, and did what they felt was right, said what they wanted to say. And damn if I wasn’t enthralled the entire time.
Tl;dr tl;dr is good game
I love Pandora. All of it. I always will.