The community was great depending on when you played it. Really boomed from its inception up until about 2010ish I'd say. Even til EoC came along it was pretty good.
Playing & competing with friends: This was another major hook of the game when it was most popular. Doing things with your friends online is fun, and Runescape was one of the biggest time-killers around to do with friends. The game was also cheap to play, or even free to play if you were content with F2p. Competing with friends in goals, monetary or skill based, generated loads of fun for a lot of people.
Clans: Clans were a lot of fun, especially in older style Runescape. Clan Wars organized by the players would see the Wild populated to a ridiculous degree.
Graphics - Some people, like me, actually like more modest graphical games. I'll even venture as far to say that I appreciate shitty graphics when the gameplay still is enjoyable - which for me it certainly was and still is to this day. Yes, it's monotonous and a grind, but the end-result (especially in the old days), was sooo worth it.
The ability to get a "return on your investment" so to speak. Most games you pay for and once you get your fun out of them, you get nothing in return. In Runescape, Gold is always valued at such a rate that you can sell off your items + gold and get a return on your playing days. I'm looking at quite a nice cash out when I'm done with Runescape. While not everyone gets this, there are plenty of people that do.
Combat system - What may be bland to you is beautifully simplistic for others. You could spice up the blandness with Staking or Pking, two things that took a good amount of know-how and strategy.
Everything was fresh, and new, and a new adventure- Minigames were jam packed, fun, and addicting. Castle Wars, Pest Control, and Barbarian Assault were AMAZING when they were released. Every single world would be packed with people playing it.
Quests + Lore is fantastic - The most under-appreciated thing about Runescape is its rich lore and great writing and storytelling embedded in quests. If you actually take the time to read the dialogue in the quests and pay attention to the storyline(s), it's fantastic. Their quests are a lot more interesting than WoW's or other popular MMO'S.
It's so different from any other game - Especially Pre-EoC, Runescape had its own niche and was great at generating content for said niche. Before it turned to cookie-cutter ideas, the game was innovative, creative, and bold. The Gower's turned a modest project into a multi million dollar success.
That's why Runescape is so successful, in my opinion.
