The freedom to do whatever I want, however I want, wherever I want, for the most part. It's just that simple. Kinda sounds like any sandbox game since I described it like that. I haven't delved too deep into other MMOs so idk if it works similar.
There's a lot that I love about OSRS but I'd say the quests and skilling are my favorite activities by far. While I always just use a guide (shoutout to Slayermusiq1), I find them way more interesting and in depth than other MMO's quests. What would be an entire quest chain in WoW could just be one quest in OSRS. As for skilling, there's just something relaxing about grinding out some stats while watching a show or just chillin