In other words, whereas a villain confronted with a sandbox world will immediately start generating ideas, Upright Heroes (typical heroes) need a plot. Without the bank robbery, Superman would just endlessly circle Metropolis, then go to work at the Daily Planet. Without the whole problem with the Ring, Frodo would just sit and hang out in the Shire forever being wholesome and loyal and sipping tea. Without fires, firemen just hang out in their firehouse, Ever Vigilant, playing cards.
I'm sort of a sucker for structural analysis of game behavior like this, but this is something that I've definitely seen in the Doom & Tea Parties game. My character in the solo game is actually pretty stand-up -- she's in the middle of a save-the-world-quest right now, sort of -- but she started out with just "I want gold for personal background reasons, and because dwarf," added "What exactly happened to the elves who used to live here?" and "My friends are in danger!" along the way, and got involved in the world-saving business in the process of following those goals. Because we started out working on other things, the she's now trying to save the world in a way that's very particular to her and interesting to me.
The thing that makes this work really well with Trollsmyth's game is that nothing in the setting lines up into Good Guys vs. Bad Guys. On a large scale, there's "the folks who want to make the world a better place but maybe have their own motives for doing so" vs. "the folks who think that 'making the world a better place' is crazy and dangerous but maybe have their own motives for working against it," and on a small scale there's a bunch of folks with their own particular things that they want out of the situations they're in. Some goblins you meet while they're torturing people for fun, and some goblins you meet while they're hunting spiders.
Which means that even if I'm going to play a fairly stand-up character (And I'm evil enough in real life that I usually end up doing that, just for a change of pace.) there's still a lot of decisions to be made and flexibility to be had in figuring out exactly what "stand up" means. Do I hire on with the priests to wipe out the murderous, troublesome pirates and make the seas safe for honest folk? Or do I go warn the pirates about the obnoxious, slave-taking priests?
This does mean that I spend more time than I would in a lot of games thinking about things like "what's the right thing to do?" and "how should society be organized?" (Though the latter is because I'm weird, not just because of how Trollsmyth runs things.) Which is all kinds of fun for me, but isn't what everyone wants out of game, and isn't even what I want out of every game--sometimes a girl just wants to goof off and steal things. But it does make for a pretty powerful, involving, and player-driven game.