I think there are two elements to this but they could be rolled into one, ‘playability’ but I want to break them out into ‘characterisation’ and ‘compulsion’.
The real answer is probably “A great GM” as that can make any shit really enjoyable just as a crap GM can make the best game unplayable but I don’t want to go there.
Characterisation
What I mean by characterisation is the ability for the player to play exactly the character they had envisioned. This is one thing that I think RM does extremely well as there are very few hard limits. If you play ‘no profession’ then there are even less limits than stock RM. By that I mean in RM2/RMC a fighter can only learn spells from open lists and up to level 5. There is no fireball or even shockbolt on those lists so now if you really wanted fireball then you needed to look at a different profession and for that you are looking at importing companion professions and before you know it you are in massive bloatiland. This is not a NP rant but the case is still true that RM really allows you to build your character the way you want it to be. There are no arbitrary rules like magic users cannot wear armour or clerics cannot use edged weapons and all that stuff.
If the players are really invested in their characters then I think they are more likely to really buy into the world and the story. If the players want to keep on playing to find out what happens then that will give the game staying power.
Compulsion
Some games or actually settings or even just campaign concepts just have to be played. They are so compelling that they bring out the best in GMs and players alike. Games like these I think come with a certain amount of ‘scaffolding’ in that the players just know how their characters should act or speak. I mean things like a pirate based game has such an iconic central theme that everyone knows how a pirate should behave, you can almost smell the salty sea air and the seaweed encrusted docks. The same can work with oriental adventures as the whole culture and customs thing is so easy to envisage. If the game experience has just the right balance or elements to make the game compelling then the players will keep on coming back.