For me, nothing. I play 1e (and have played 5e et al). Pick the one that gets you excited. If you like the art, the names of the monsters, and the community, those factors outweigh any minor differences in rules or organization.
It's similar to programming languages - there's an ineffable quality to a language that makes you just want to use it (or not). I knew someone who picked up DarkBASIC several years ago; I could have suggested he get into Python or something more "mainstream", but I think the right choice is the one that makes you want to sit down and work on your project.
It's similar to programming languages - there's an ineffable quality to a language that makes you just want to use it (or not). I knew someone who picked up DarkBASIC several years ago; I could have suggested he get into Python or something more "mainstream", but I think the right choice is the one that makes you want to sit down and work on your project.