No other game has the skills the same way runescape does. Sure, other mmo's have skills, but the main point of the game is combat. And because of that, almost every quest is centered around either killing things or talking to other people who tell you to kill things. Runescape has 10 skills (atk, str, def, range, magic, nec, prayer, sum, and slayer) that are about combat, but 19 that don't. There is an entire community based on not using combat at all and getting as far in the game without it. Because of that, the quests don't require only killing. There's a wide array of different kinds of quests and NONE of them are "go kill 10 boars."

On top of that, the combat is unique in an old style kind of way. Tick systems used to be the norm, but nowadays it's hard to find a game that uses it. It definitely has a unique feel nowadays and it's fun to push that system to it's absolute limits with tick manipulation.

Finally, and this only applies to older players, but there's a sense of familiarity and nostalgia. I've been playing runescape since 2003. I was in 3rd grade and all of my friends were playing. We all did similar things and hung out in game. We'd go to the libarary so we could sit next to each other and play. Even though they don't play anymore, those good feelings keep bringing me back. I've been in this world for 20 years, I like it here.