- Kill enemies (like Space Invaders)
- Collect items (like Pacman)
- Reach Location (like Mario - although he also collects items and kills enemies)
Even a game like Gears of War is basically made up of just these goals - get to the extraction point while picking up weapons and ammo and killing locusts. But that's not what makes GoW exciting or fun - it's the obstacles that get in your way - environmental puzzles to solve, enemy behaviour to study and overcome, and the twitch skill you need to survive.
The problem with RPG quests, like you find in World of Warcraft, is that they often remove all the obstacles, puzzle solving and skill that GoW requires, so that all that's left is repetitive"grind" - fighting, collecting and trudging around, without any real thought going on. If the quest designer has the tools to make an interesting journey for the player, they can make something rewarding - if they are only able to define basic kill 10 rats scenarios, then the experience will be a chore.
It's worth noting before I get into this topic that "killing" doesn't really have to be as violent as it sounds. Killing an enemy can be as metaphorical as you want to make it. Maybe your enemies are only stunned, maybe you're freeing them from some king of mind control (wasn't Sonic like that?), maybe they're not really enemies at all but customers in your cafe that you need to serve. The gameplay effect is the same: you have tapped into the part of the human brain that has evolved to deal with understanding the behaviour of another sentient creature, in order to get what you want from it, be that to eat it, not be eaten by it, get it's money or win it's affections. In The Sims for example, aren't each character's enemies really their neighbours and housemates? They must be charmed and seduced in order for the character to get what they want from them. Failure to do so will send your character into a spiral of depression, which is ultimately the closest The Sims has to losing.
Each type of enemy should have a unique appearance, but most importantly should have a unique behaviour, way of moving and interacting. Encountering an enemy for the first time should be a mini-puzzle, where the player must figure out how they need to manipulate their avatar in order to avoid the enemy's attacks and hit with their own, even if that mental process happens deep in the reptile brain rather than through a concious decision. If the player has different types of weapons, abilities, special moves or unit types to control, then each should have a different effect and effectiveness against a particular enemy. Real time strategy games do this really well, where, for example, a commando can take out a person in a single shot, but is useless against vehicles, forcing the player to think strategically about which enemies to attack with which of their own units.
Collecting items can be for its own sake, or it can be because they confer some ability that is useful in achieving the player's other goals. A game like Farmville uses only this goal - the aim is to accumulate as many coins, crops and animals as possible. There are is no location to reach or enemies to kill or avoid along the way, and you are never in any peril. The only thing you can do wrong is to not login to the game to harvest your crops, but this doesn't require skill or strategy, only a tireless devotion to the game. This is how Farmville is so successful in attracting a non-gamer audience. The player is only rewarded and never punished, unless they decide they have something better to do.
In a game like Gears of War, the main purpose of items is to make the player more effective in killing enemies. A gun or ammo pack has no sense of worth, other than as a means of eviscerating the enemy, and a player will happily toss it aside when something else comes along. Now compare this to an RPG where a weapon also has a monetary value and might have been difficult to acquire, and you'll find that a weapon becomes much more a prized possession. RPG's are very materialistic in this regard - every item has a price and the player spends a lot of time studying and organising their "inventory" and shopping for new items. As in The Sims, there's a lot of playing dress-up/playing with dolls here, and this is something which actually appeals to boys as well as girl gamers.
Collecting items can also be used as a way to test the players skill at the game and knowledge of the levels, with items placed in hidden or hard to reach places. Even Gears of War has it's Cog Tags, which serve no purpose other than to demonstrate that a player has a trawled every virtual inch of the game, which brings me onto the next topic...
Since the earliest recorded times, stories have used a journey structure, where the protagonist sets out to reach a particular place and has an adventure along the way. Humans evolved to be semi-nomadic, and even the biggest homebody has a wanderlust somewhere deep inside. We love to travel, or climb trees or see what's around the next corner, even if we have no need to. Ever since the technology has permitted games with a playing area larger than a single screen, videogames have used this urge. But we tend not to make it too easy for the player to go exploring. Aside from enemies in the player's way, there are also environmental obstacles. I personally get nothing out of having to perform a set of perfectly timed jumps on every screen, but there are also environmental puzzles to test the player's logical brain, as you see perfectly executed in games like Half Life 2 and Portal.
So that's my breakdown of what I see as the 3 main goals a player has in a typical videogame: kill enemies, collect items, reach locations. Or if you wanted to be really reductionist, you could say that the goal of games is to keep some objects far apart and some close together.
Go any thoughts on this? Post them in the comments.