My first thought was of quests. They RuneScape quests usually do a good job at RS Gold having a sense of excitement and interest instead of boring and tedious (note that I said that often). ...). Certain quests are only for exp rewards, but many offers significant unlocks or unique things (e.g. keyring). Thus, the questing in RS feels different from similar techniques in any other RPG I've played. It's also helpful that they don't impede on the story of the game by siding quests. Instead, the primary story is told through all these stories that are scattered around the map.
After thinking about it some further and reading other responses, one thing that I think RS does well is offering an array of both intense and casual gameplay. The majority of other games tend to be mostly either or both that limits how often I am willing to play. The fact that I can play AFK Sand Crabs or NMZ when I'm working on something that will help me train to a certain extent and then go to a boss game later when I can focus completely on my game. This is something that other games don't offer and isn't available in RS.
One example is that the other MMO my friends and I are playing GW2 and although I think it's an amazing game with a great combat system, I'm unable to find a single thing in that game that is even close to "AFK" similar to what is available in RS. If I'm playing it, it will take up most if not the entire screen and demand the majority of my attention in comparison to OSRS which is a small window in the corner which will only require me to look at once in a minute or at most. Most likely not the kind of gaming that most MMOs wish to promote, but when coupled with an engaging and engaging game, I can see it as an asset.
Items within OSRS have real value. They can be dropped on the ground and picked up by others and have multiple uses and so on. A bucket functions as the same bucket no matter where you get it from. Other MMOs which require items such as buckets for quests, for example, give you an icon with an image of a pond in it once you have accepted the quest. You can click on it to finish the quest, then the item disappears.
Additionally it is important to note that the OSRS world is designed to resemble real life and not a linear set of zones that you must complete and never return to (like in many MMOs with theme parks). This means that you'll encounter players of cheap runescape accounts all levels everywhere you go, doing their own thing, since each town has many different applications. This makes the game more lively.