(This is the second part in a discussion of why a game world must be realistic. )
Once players know what their actions are likely to create, they need to care about those results.
Engagement (caring about the quest)
In a world where NPCs exist to a) advance the plot and b) be killed, it's hard to make your players actually emotionally connect to your NPCs.
NPCs need to have lives outside of their interactions with the PCs. This doesn't mean you sit around and roll dice when your players aren't there, but it does mean you need a realistic world. That NPC needs to have a reason why he is the level he is and why he is doing the job he is doing. Your players need to believe this, and you need to be able to create it on the fly when they start asking questoins. To that end, you need a set of rules that produce results logical enough for you to interpolate from.
I've gone through great lengths to explain why laborers make 1 sp a day while adventurers make 1 gp a day. I've worked out the economics of farming, including what happens to the tax base when some smartass starts casting Plant Growth everywhere. Fortunately you don't have to; you can just download my campaign materials. But WotC really should have done that in the first place.
Once your players realize those laborers are working as hard as they can, and their low wage is a result of socioeconomic factors beyond their control, then those laborers become more like people and less like scenery. When the guards are as tough and effective as time, money, and a burning desire to live can make them, but are stuck at 1st level because of iron-clad rules of physics (rather than being genetically "incapable of progressing upwards"), then your players will respect them more. When the King uses his levels to defend his people against monsters, then your players will think of a throne as more than an excuse to make the rules and collect the taxes.
And when they ask why they can't get rich casting Remove Disease for 150 gp a day, you'll have an answer for them. One that makes them try harder to invent something new, rather than giving up and accepting DM fiat. This will spark creativity, rather than sessions of "guessing what the DM will allow". It also means they won't consider the fate of the NPCs at the hands of the monsters to be DM fiat either.
Responsibility is the heart of engagement, and your players need to feel responsible for the world they play in. This means the NPCs need to live up to their responsibilities, too; otherwise your players figure out the game is rigged, and they stop caring as much.