(This is longer than I intended. :P )
A player plays a game. As the player experiences the game, the software builds a complex structure of ideas (gameplay, graphics, story, etc) which interlock and support each other. The tighter and smoother these interlocks are, the tighter the game holds together. If you know what a memeplex is, it's essentially that. Regardless as to whether you know what that is, think of this as legos. Each moment of play is another tiny lego, building a giant final structure.
"Yeah, duh, games build on themselves. Who cares?"
Well, at a very basic level, how good the ending of your game is depends largely on how well it "caps" the structure you've created. Some endings which should be good endings flake out because the legos don't support them. Some endings which should be trite and bad end up quite touching because the underlying structure is built specifically to need only a little bit of additional work before the structure completes.
But how to think about your endings is only the tiniest piece of this puzzle. After all, this same basic idea applies to comics and movies and books and classes and business deals and blogs and...
In a single-player game, you'll see that a game builds its structure up much like a TV series might, although using pieces that no TV series has access to (like play).
But in multiplayer games, things can get very complex.
Imagine a tabletop RPG. Six players and a GM. The GM is simulating the world for the players. He is placing pieces and creating a structure. However, any given event usually focuses on one or two characters: this round, Anna the Angel and Bob the Barbarian got hit by a squad of storm troopers. But Cinnabun the Ranger and Drokmok the Dancer don't care as much about that.
Essentially, the GM is trying to build a complicated, tight lego structure for each player, but when he puts a piece down, he can't tell how long it's going to be! It's different for each player depending on how much they care about the event that just happened.
There are three ways to deal with this: loose structures, corrective applications, and self-construction. I bet you've never heard of at least one of these concepts before:
Loose structures are the path most GMs take unconsciously. They quickly learn to create lots of "slack" in their game structure so that players can do various things and feel various ways without making the whole game collapse like a Jenga stack. This does end up creating a structure that looks more like lace than a sturdy wall, but it works okay. It's not very efficient at creating a good game, however.
Corrective applications are not quite as common. When a GM does this, he gives another lego piece to players which don't have enough support. Essentially, he "fills in the holes". Like building a lego structure, you should fill the holes before you cap them. This usually means that the GM does one-on-one mini-sessions with the players who feel left out, or makes sure that roughly the same number of interesting things happen to every character in every session. This isn't the best way to do it, but doing it better would require an astonishing amount of micromanagement.
The third path is the path of self-construction. This is a pretty rare path for "mid-level" GMs to take. A lot of early GMs take it and a lot of advanced ones do, too... but it's too common for a GM to hold on to his game too tightly to allow for this kind of freedom.
In self-construction, the GM sets up parts of buildings and then lets the players try to put them together in any reasonable way they can think of. Players usually cooperate to glue them together into structures of incredible density. The GM is no longer really a Game Master, but a Game Guide: he gives the players new pieces if he thinks they're running out of bits to glue together, and occasionally kicks down someone's building.
The downside to this method is that each player will end up with a different building, meaning that as the game progresses it gets harder and harder to give everyone pieces that fit into their own unique structure. This can be controlled somewhat by an experienced GM, but always happens. It's especially bad for endings: typically, these games require half a dozen individual endings wrapped up into one giant blob.
"So I should think about using a loosely defined progression, keeping all my players happy, and letting them do their own thing from time to time? Gee, thanks, I would have never thought of that on my own!"
In my opinion, each of those methods could have a book written about them, but for now I don't have time to further extrapolate because I'm jumping into...
MMORPGs are kind of the ultimate example of how to do all three of these things. MMORPGs allow players to get as involved as they like with whatever they like whenever they feel the need. It lets them go to whichever piece of structure they want and claim it into their personal story. It doesn't hold them to a specific plot.
Actually, MMORPGs fall short in two ways. First, they don't really allow players to self-construct much. The players can't really build their own buildings very large, save through guild mechanics. There are other ways I would consider superior.
The second way MMORPGs fall short is that they don't have very much in the way of a Game Master (or Guide, or whatever). Essentially, there's nobody who sees a particular player and goes, "they really need this particular piece of building." This lack of guidance means that players get pieces of structures, all right, but none of these pieces have much chance of ever fitting together.
Is there a solution? Well, maybe you could record the "shape" of each piece of play, and then keep updating a player's profile to see what kind of shape might go nicely. It would, however, be quite rough. Worse, it would probably be hard to correctly guess how much time the player wanted to spend this week.
Barrier to Entry
Most games strive very, very hard to lower the barrier to entry. Generally they do this by having an introduction which forces any player who joins to go through a tutorial. This lets the game build a lego foundation so that the player, when he starts receiving pieces of building, has something meaningful to attach them to.
For one player games, this is pretty simple since everyone starts from scratch. For MMORPGs it's actually quite hard, but they spend a lot of time and money on it and it works out.
Now, how about for a tabletop?
Ever tried to bring a new player into a tabletop only to realize that he doesn't fit? This is especially true of my games, since I build rather complex structures and I build them vertical. So a player comes in and I'm giving people pieces that fit into this one weirdly-shaped little node that they all share fifty stories off the floor. The new player gets this thing and just stares at it. It can't even stand up on its own, let alone be used as a foundation for further play.
Chances are, your games are a little less... vertical. But you probably suffer the same problem.
The obvious solution is to have an introductory session with just them and maybe one or two other guys, so you can build a foundation that makes these pieces work out okay. These kinds of sessions work well, so long as you build fast and strong rather than trying to rebuild the same building the other players have. The new player will still occasionally get pieces that don't fit, but if you're running with the player-construction method, everyone occasionally gets pieces that don't fit.
The problem with an introductory session is that it takes hours and hours. If you run games like I do, it is fundamentally impossible to have that much time. If I spent even an hour with everyone who would join Boogaloo if I only laid them a foundation, I would have to spend all my free time doing just that. Yikes!
For my grander schemes, such as an on-line version, this is an even more ridiculous idea.
But... what if...
Some GMs have assistant GMs. I generally don't, because it's impossible to tell them enough to keep them up to date as to which players need what kinds of weird structure pieces. Of course, this means that I have a player maximum (functionally about five players, despite the fact that my games generally have two or three times that). There's only so much attention I can spend.
The place you're most likely to see AGMs is in large LARPs, where they are charged with mostly just making sure people know the rules and arbitrating when the rules don't apply.
But, really, is there any reason to have them be AGMs? Can't players arbitrate and teach just as well? Sure, the player may be biased, but once word spreads as to which players are bad at it, they won't get asked to do it any more.
Can't this be used to create a kind of "pryamid scheme" for teaching new players? Instead of the GM spending time, a player spends time. This lets the game scale functionally infinitely.
At that point, the real question is how to track so many player's needs... or to automate their ability to create pieces that fit those needs.
Bleah. Sorry, this was long.