Many of my posts talk about how to keep players entertained. One basic idea is that if you throw variation at them, the players will be more immersed. Whenever a player starts to get "used to" something, you twist it or give him something new to chew on.
There are really three kinds of player interest, as far as I can tell.
Statistical is the most common one. That's when a player is caught up in the numbers and algorithms of gameplay. This version is fed by a new weapon or a level-up.
Visceral interest is the second most common one. This is one where players enjoy the results of their play on a pretty primitive level. For ease of use, this covers everything from a great death animation to succeeding in a long campaign. Even though the difference between these actions is tremendous, the end result is a "Yeah! Awesome!" feeling, and that's the feeling I'm talking about.
The last and least common type of interest is adventurous. This is often the feeling that makes you explore every building in a town, or talk to every character in the game. The idea that there is stuff out there - exciting new fun stuff to get and learn about.
Adventurous interest is the one I'll be talking about, because the other two are pretty easy and have already been covered. However, some of the details here can apply to all three kinds of interest.
Most games start out with a lot of adventurous interest. The first level or two, you'll want to explore every crack and see everything there is to see.
Why do some games quickly and quietly fade into non-adventures? A lack of reward for exploration? High cost of exploration?
Well, maybe a little. Let's take a quick look. Three very adventurous games are Chronotrigger, GTAIII, and System Shock II. In each of these very different games, there is an urge to explore everything.
Statistically, only Chronotrigger actually rewards adventure. It gives out treasures. GTAIII doesn't really give out meaningful rewards for just going to some random part of the city for kicks, and System Shock II actively hoses you for exploring.
Which means that we're probably using the wrong definition of "reward".
GTAIII does reward adventure. It allows you to play in a wider variety of spatial set-ups, whether on foot, car, bike, boat, plane... this variety is the reward. The awesome jump ramp, the high-performance ferrari, the ice-cream truck.
Although SSII does give out rewards (a few grenades, a new pointy stick), the cost of getting them is usually quite high and any given character can only use a quarter of what they find. However, SSII also hands out information: dozens of tiny plot morsels and backstory tidbits. That makes it more worth it. The reward is not only nontangible, but not even play-related. (It also rewards with interesting play spaces.)
Chronotrigger rewards not just with prosiac item treasures (ooh, my 500th "potion") but with fun NPC dialog and minigames. Sometimes, the NPC dialog is just entertainment, sometimes it's plot tidbits.
Look at some of the more boring games. They might reward exploration with another treasure, but without the variation to back it up, it feels boring. You have statistical interest - robotically exploring the town to maximize your resources - but no real joy at finding some new gem. (This would be mitigated if the games were harder, in which case visceral joy is added to the statistical joy... but it still isn't adventurous.)
The difference, to me, is obvious: variation in rewards.
The thing I've never mentioned before is that there are two kinds of variation. You can think of them as deep and wide, if you like, but I prefer "stacking" versus "transient".
Stacking rewards are a new weapon and another potion. A new version of the same old. "The same old" is hardly adventurous. To be adventurous, you have to be stumbling over new things. But it's more than that:
Stacking rewards are used in gameplay, and either never get used up or simply aren't used up fast enough to really push the player. This means that you have a steadily increasing number of play options. To keep giving "new" stacking rewards (a machine gun instead of a rifle, a rocket launcher instead of a machine gun) you essentially make the old upgrades obsolete. So the player is carrying around things he'll never use, and the new things aren't "new" at all - they're just statistically better versions of the old things.
What can you do?
Transient (or "wide") rewards are ones which do not increase the number of play options. They can do this by either not being about play options at all (plot fragments, NPC dialog) or by replacing other play options (minigames, new spatial layouts).
People talk about Halo as a very good game. They talk about what they liked about it, but none of that stuff actually stands out. The level design was boring. The weapons were pretty prosiac. The combat balance was a little flubbed up, with broken dominant strategies hitting brick walls when the player could least afford it.
When people are playing Halo, they may all have different complaints. But one complaint everyone shares, every single person says, is right at the beginning of each level. "Goddammit, they replaced my weapons with crap!"
The reason Halo maintained interest was because you could only hold two weapons, and there was usually very limited ammo for them. Transient rewards: if you want a weapon, you have to ditch a weapon. When you start disliking your weapons or running out of ammo, you'll find yourself avidly exploring the level looking for a good weapon.
Transient rewards should work just as well in tabletop games as computer games: my own experience shows it works extremely well in LARPs and board games.
So, think about it. Instead of giving players new options or a flat upgrade, keep the number of options the same. Give them plot, humor, a replacement that doesn't work the same way, a layout or monster with some weird quirks.
It usually requires more effort - creating new plot fragments and dialog - but as Halo shows, there are ways around that. I don't think Halo's "weapon selection trick" works as well as continuously NEW experiences. It only "activates" when you want another weapon: there's only a mild urge to explore a level when you're not in trouble, and when you are in trouble it's too dangerous. However, it is somewhat effective and it is cheaper.