Hi,
If motivation consists of a few components, then thought line perhaps corresponds to the action taken in the world. Most of us play games to "have fun,"but the action taken in the world that fulfills the passive motivation might be something like "build a massive civilization"or "crush my opponents" or double quote do something very clever with the rules and we in in a surprising way.
I have conflated actions in the real world with actions in the game world, and I think that is fair. Because, if we limit ourselves to the real world, then nearly every role-playing game had a thought line of write up character sheets and roll dice while eating crap that's going to kill you in a few years! I suppose it's fair to say that a good core mechanic is one that is either fun in it self, for whatever reason, or nicely mirrors the actions taken in the game world, or is extremely unobtrusive.
"but we are talking about" thought lines. (Whoa, Dragon did something interesting to this line!)
I think that the general popularity of the game goes hand-in-hand with the simplicity of its thought line. Dungeons & Dragons is easy. Kill things, take their stuff, become more powerful, rinse and repeat, and be applauded for this. Vampire is also easy, though not quite: be cooler than you are! Kill your enemies! Feed on your inferiors, and take their stuff (unless you are too cool for that)! Wallow in angst! lay low when the herd of inferior beings who cannot possibly appreciate your awesomenessis too big to handle, or is too powerful in a totally uncool way!
Then there are games like Nobilis. Most people don't know what to do with this. The "you are" part is easy, character creation is also easy, but now what ? Ars Magica is not quite that difficult, but I don't think there is a slot line to the game. Or rather, there are many, depending on the saga, depending on the player. Do you want to track a medieval village in Excel? Do you want to bend the rules into a pretzel (me! Me!)? Do you want to participate in real medieval history (well, sort of)? Do you want to kill monsters and take their stuff? Do you want to meditate upon the ultimate loneliness as you decay into twilight or death or something? you can even have something like remake Europe according to your whim, the only thing stopping you are the other players. many games are essentially social games: design cool spells! Create things in the world! Get the other players to accept your stuff!
I also note that there is a class of game – of which are as Magica might be the progenitor – in which the "thought line"is severely reined in. For example, whereas, in Dungeons & Dragons, you are totally encouraged to accumulate power and use it, in ours Magica, increasingly, there are restrictions on how power can be used and how powerful it really is. If you use your power against other wizards, you die. If you try to use your power against Angels, you die. If you even look at other wizards with your great power, you die. Your magic won't let you noticedemons, which is too bad, because you'd really like to. you can live for a very long time, but probably not too much longer than people live today. And so on.oh, and if you try to do something to cool, role botched nice.
The most successful game of this kind is vampire, in which your awesome powers don't start off very awesome at all, and in which if you use your powers and call attention to yourselves, you get squished. People understand this viscerally, because everyone knows that vampires get hunted by vampire hunters. But the other games in this series come up with similar restrictions, both from human society and from the specific supernatural society and from the designated enemies. Mage is the most obvious about this: you have awesome power in theory, but if you actually use it you will get squished by Paradox and the technocracy. The exalted series, to some extent. Even Nobilis.
In a sense, the thought line for these games has to be more complicated, because the awesome power is deliberately crippled. this is not necessarily a bad thing, but I believe it is a true thing about these games.
Anyway,
Ken