It's hard to escape the concept of repetition in entertainment, especially in gaming. While the word "repetitive" itself is often used as a criticism (hello Halo 1's interiors) games are designed to be played repeatedly, and incorporate repetitive elements into their designs.
It's understandable. As an object lesson in entertainment economics, look at the DVD player. Widely hailed as the fastest-adopted new entertainment technology, it is built on the foundation of repetition; the idea that people will want to play the movies and television shows they love over and over. Given that game console hardware and software are both about three times as expensive as DVD players and DVD discs, they have to be at least as repeat-friendly to warrant that kind of investment.
There a lot of different ways to extend a game's useful lifetime and give gamers more bang for their buck by allowing for repeat plays; the Halo series, as well as many other games, provide excellent examples of this.
Since the nature of online multiplayer itself is repetition-- short competetive matches played with a seemingly endless revolving door of random opponents-- we'll leave that aside for the moment. Many games don't have multiplayer at all, and even most that do don't encompass all of their purchasers in online matches. However, there are many ways that repetition is used in designing a single-player campaign that can remain interesting after many playthroughs.
Harder, Better, Stronger, Faster
Perhaps one of the most-often used ways of adding repeat-play value is through offering multiple levels of difficulty. Of course, such levels are also useful, if not primarily useful, for allowing the first-time player to select a way of playing the game that suits their style of play so that the game experience will neither seem dissapointingly easy (and thus too short) or frustratingly hard, and therefore unfinishable.
Once you've played through once at your preferred difficulty level, though, you can try again at another level; a harder one if you want more of a challenge, or an easier one if you just want to play around. Creating these different levels in Halo and Halo 2 primarily revolve around changing the composition of enemy encounters, and adjusting the damage model. Want more of a challenge? Put in more of the harder enemies-- spec ops units, Hunters, Elites. Want to tone things down? Fewer units total, and more of them easy targets like Grunts. Give each unit less or more health, making them easier or harder to kill, and change the weapons so that they deal less or more damage to the player, making them less or more of a threat. While changes in behavior can also be made, if we're to believe Bungie's AI lead programmer, Damian Isla, these are mostly small things like the rate at which units fire grenades; something he mentioned in a recent Bungie Podcast. Most players have the perception that the AI characters are smarter at higher difficulties, but apparently this has more to do with being hardier and longer-lived, thus allowing more time for the expression of AI behaviors, than actually being smarter.
For some game designers, just offering multiple levels of difficulty isn't enough, so they take it a step further. Shooters like Doom, Gears of War and Black conceal their harder difficulty settings until you've completed the game at a lower stage. One might say that this is to prevent a gamer from being frustrated at a high difficulty level before they've familiarized themselves with the game, but I think the real reason is not just to allow repeated playthroughs, but to encourage them; present the player with only Normal difficulty, and then taunt them with Hard once they've finished that. It's an open challenge to the player, the equivalent of the designer saying "well sure, you finished the game on Normal, but are you good enough to beat Hard?"
The Halo games don't do that; and it is possible to take a good thing too far. Black, for instance, is almost unseemly in the ways in which it attempts to get you to replay the game. When you start a game for the first time, you have only two difficulty choices: Easy and Normal. The implication is that Normal is for real gamers and Easy is for pansies. Completing the game on Easy gets you nothing. Completing it on Normal does two things: it unlocks the Hard difficulty level for each level completed, and completing the entire scenario unlocks the "Silver Weapons" feature, but only for levels played on Normal difficulty. Silver weapons have infinite ammunition.
In other words, having played through the scenario once on the Normal difficulty, Black is now offering you two different incentives for replaying the game now, not once, but twice! If you want to see what the silver weapon feature is like, and romp through the game without worrying about ammunition, you can do that-- but only on the Normal difficulty. Then, of course, there's the Hard difficulty setting now available.
Completing the game on the Hard difficulty does the same thing again. Now you've unlocked silver weapons on the Hard difficulty, and unlocked another level of difficulty-- "Black Ops". Complete that level and you get silver weapons unlocked for the Black Ops difficulty level.
In other words, if you discount the Easy setting, to completely experience what Black has to offer in terms of difficulty and infinite ammo sandbox play, you must complete the single player scenario a total of six times; once on each difficulty level, both with and without silver weapons. Neither of these changes make much difference, either. As with Halo, mostly what you get are more enemies and tougher enemies. Even before Black Ops it's gotten tedious; enemies even without head protection seem to take multiple headshots to eliminate, and ones with helmets and riot shields survive direct RPG hits while dealing major damage to you from a hundred meters away with a revolver. It's a bit silly.
Alas, Poor Yorick
A much more clever way to encourage repeated play is shown in Halo 2, in a feature that should be greatly expanded in Halo 3. Rather than just allowing for more enemies or more ammunition, the hidden Skulls in Halo 2 changed many aspects of gameplay: gravity and physics, ammunition pickups, grenade-throwing behavior, shields, melee attacks, explosive headshots.
Halo 2's skull feature did abuse the player in one respect, though; in a Black sort-of way, it forced the player to repeat whole areas of levels to get them. Once picked up, the skull's effect only lasted until the console was restarted. And since you can only have one saved checkpoint per profile, players either had to first go and collect each of the skulls they wanted on their respective levels before choosing the level they wanted to play with the effects active (all on Legendary difficulty, mind you, because skulls aren't present on others, except for Blind) or create multiple profiles, each with a saved checkpoint near a skull, and then switch between them; also a bit tedious. Here's hoping that once located, there will be a more convenient way to mix and match skull effects to vary your multiple playthroughs of Halo 3.
One way to create small increments of replayability within scenarios is used in Halo: randomization. Instead of having static enemy counts within encounters, Halo difficulty levels have ranges; so even on the same difficulty, the same encounter might play out a little differently by changing, oh, say, one Jackal for two Grunts.
Some encounters in Halo, specifically on Assault on the Control Room, take this a bit further and randomize a bit of the terrain, notably on the last land bridge. The game gives you an active camo and encourages you to run the gauntlet out that last door and towards a waiting Banshee. Just when you think you've got that run down pat, you load up the game and see a different configuration of rocks in your path, forcing you to improvise. It's a small touch and is only present in certain particular points, but it has its effect.
I'm Ready For My Closeup, Mr Staten
Right off the bat, Halo 3 offers you roughly twice as much play time as just about any other shooter. Why, you ask? Saved Films.
I've often thought of just about every video game as essentially a puzzle that rewards you with digital art when you solve it. The ultimate reward for solving a level is seeing the next level; seeing the next piece of art.
Of course, if there's a problem with how this is accomplished in modern games, it's that there's simply so much art and so much action going on that you can hardly appreciate the former for an excess of the latter.
Saved Films solves that problem. Ever got killed by a hidden enemy while you were admiring the scenery? Probably every appreciative Halo fan has. Now every Halo player can relive those moments, from any perspective they like, with the Saved Films feature. Most fans seem to perceive this feature in terms of multiplayer-- being able to observe themselves and the competition after a match and thus learn from mistakes and from new tactics. That's a perfectly good use of the feature. Or, they think of it in terms of simplifying the process of making machinima, which is also often done in the multiplayer modes. Saved Films also work in campaign, though, allowing you to freeze the action, move the camera, set up screen captures, and observe all the gorgeous vistas you were too busy getting shot at to truly appreciate while you were playing. Want to watch a huge pitched battle play out from the perspective of your allies-- or your enemies? I know I do. Up to now, players have been the Master Chief, but they only get to see him in cutscenes, which almost never feature action. Now, you'll be able to replay a saved film of that campaign level you just played, detach the camera, watch the Master Chief go to town on a bunch of brutes and say to yourself, "that was me." Plus, you'll be able to host that film, or clips from it, to show your buddies.