Retail lets you log on and follow a story. Vanilla let you log on and create a story.
Currently, the game starts at 110. Everything you do before is pointless because you level up so quickly. There is no point putting more than 5 minutes into getting an item, because either you already have a heirloom or it’s replaced within minutes.
Vanilla the game started at level 1. You could happily do an hour long dungeon for a quest, because the reward would last you days of /played time. I’m currently level 51 on a private server, and sat at around 9 days /played. I haven’t felt like it was a slog at all, because there was always something to explore, or some group quest to fight through with a group of players.
The social aspect is like nothing in retail at all. You make a name for yourself and get remembered. Me and Tuto got asked quite a few times to tank and heal dungeons because people remembered us from the past when we did a run with them a week ago. Most people on retail can’t remember the name of the tank from the dungeon they ran 5 minutes ago, because it just doesn’t matter.