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.
This is a very similar question as the previous ones. Or my answer is at least very similar. It has to be a game that everyone enjoys. The good times and comradery comes out and people have a great time. The game can’t be overly complex or take forever to complete a session. Axis and Allies took a long time to play and there was a WW-II game that took forever and people had to stop where they were and come back too it next week, but it was a board game!!! RPG’s have natural break points and goal completions, that’s not quite the same as a board game taking days to play.
Yes, the GM is absolutely the spearhead. He needs the ability to make the game system fun, the imagination to make the settings interesting, and the personality to keep everyone rolling in laughter.
I’m going to guess that the board game is The Campaign for North Africa (it cropped up in an episode of The Big Bang Theory). Estimates to complete a game are 1,500 hours. For ten people.
There are any number of “super games” that can fit that bill, honestly. Some of SPI’s games were designed to be linked and played in that fashion, and there were other designers who exceeded even that.
To me what gives a game staying power is flexibility. By that I mean the core rules are flexible enough to allow what Peter mentioned along with setting changes without huge conversions and other modifications. Flexible rules allow players to develop the character they envision without having to rely on canned “archetypes” or other tropes (some I’ve seen have well over 20 or 30 of those…not conducive to play as far as I’m concerned). Flexible rules also allow the GM to shift things as needed without breaking either the game or the mood.
Flexibility can mean many things. Sometimes it’s Peter’s treasured No Profession, but other times it’s a clearly and cleanly developed set of progressive careers like Warhammer’s (along with the ability to ignore those and just jump from basic career to basic career, which the system also allowed). It can also be the ability to change profession with minimum fuss and bother. It’s the ability to drop a set of rules in and add firearms to any setting, or lift out magic without the system exploding on itself.
When a system loses flexibility it either has to have an outstanding setting or simply have arrived on the scene first (I’m looking at you, D&D).