I've recently been playing Assassin's Creed again. In virtually every discussion about the game there's one thing that almost always gets mentioned by at least one person - the game's mission structure is too repetitive. It's a criticism that I agree with. For all the things the game does well - and there are some things that the game does do very well - in the end it feels like you're more or less doing the same thing nine times.
But at the same time, there are games that I love that are essentially built on repetition. Between EA's NHL series and 2K Games NHL 2K series, I've played I'd guess over 1000, maybe 2000 games of video hockey over the past 15 years or so and logged a few hundred hours in the process. (And as an aside, it's kind of scary when you break down your gaming habits like that, isn't it?)
So what's the difference? Why am I willing to play the same game of hockey a couple hundred times but feel ripped off when I have to perform a pick-pocketing mission for the fifth time? I think the main difference comes down to a topic frequently discussed on this site, narrative vs gameplay.
Repetition in Narrative Based Games
I think the main reason we get bothered by repetition in narrative based games is because it breaks the narrative. The team that made Assassin's Creed's Holy Land spent a lot of time creating detailed, believable, life-like cities. They set up a potentially interesting story spanning multiple time periods and covering some interesting topics like the role of pharmaceutical companies, the motives of invading armies, and the roles of memory and ancestry. And then they throw you into that world and ask you to do a bunch of standard video game fetch missions.
This breaks the player's expecations for the game. The player wants to push the story forward. The player wants to become a part of the world they've been placed in. Instead, the player finds that what they thought might be an interesting, believable world is actually just a set-up to get them running through a city full of generic soldiers harassing exactly the same citizens in exactly the same way, requiring exactly the same techniques to defeat them, and producing exactly the same results. This produces the exact opposite of what the game wants - the player feels like they aren't a part of the story or the world.
The end result is that the game feels too much like, well, a game. Your actions don't feel like they're part of a world or a plot, they feel like arbitrary obstacles placed in your way because the game needed to provide you with a set of challenges to complete.
Games with a heavy narrative element succeed largely by creating a varied and constantly shifting set of tasks for players to complete. In the Metal Gear Solid games, virtually every room has you using your skills in a different way than the previous room did (the Legend of Zelda games, not really narrative in the same way, are also an excellent example of this type of design).
In the Grand Theft Auto games, even though the core gameplay of stealing cars and driving around can be somewhat repetitive, the variety of tasks that you're given to do within that framework is actually pretty huge (and San Andreas may have even taken it too far, providing more things to do than most players can likely manage).
In God of War, individual enemy types require different strategies and move sets to defeat and behave in different ways, and the structure of the levels themselves change a fair bit as well. In narrative games, players need to feel like what they're doing matters, and repetition breaks that.
Repetition in Non-Narrative Based Games
If all of that is true, though, doesn't that mean that repetition is bad and that games should avoid it as much as possible? Not necessarily, and I can think of a few genres that actually clearly benefit from repetition. Sports games, racing games, and fighting games all immediately come to mind as games whose actions are highly repetitive and are better, not worse, for it.
One reason for this is that they are all in a very literal sense games. That is, they are competitions under a set of clearly defined, mutually enforced rules. All cars on a race track have the same victory condition - completing the required number of laps before any other car does. Both teams in a baseball game have the same victory condition - score more runs in nine innings than the other team.
But the characters in a game like Assassin's Creed do not neatly break down into opposing factions, and even those who do (like Altair and his targets) are operating under very different rules and victory conditions. Assassin's Creed is not really a game in the traditional sense, so much as it is a story that uses some of the trappings of a game.
And since sports, racing, and fighting games are all games in the literal sense of the term, they are not only suitable, but actually designed to be repeated. This allows players to develop their skills as part of an ongoing process of learning and refinement over the course of many plays.
An individual hockey match is in one sense a complete game, but it is also part of an ongoing process of developing skills and strategies to compete in other discrete hockey matches. In a narrative game, losing breaks immersion and causes the player to have to re-do sections of the game they have already completed. In a hockey game, losing is not only accepted, but expected. This is largely because a loss, especially early in a season, has very little impact on the player. It's a learning experience, and if the player wins the next three games then it doesn't really matter.
I think that's the main difference between narrative and non-narrative games in terms of why repetition damages one and improves the other. In a narrative game it is extremely important that the player feel like they are a part of the world, and repetition can often break that. In a non-narrative, competitive game, repetition is actually a part of the basic design, and is intended as part of a process of learning and skill definement. The truth is that repetition isn't bad for games, it's bad for narrative.
One type of game that I can't fit into this model, though, is JRPGs. Games like Final Fantasy and Suikoden are popular largely because players are so involved in their characters and stories, and yet the underlying gameplay is as repetitive as that of virtually any other genre. I think I have a decent explanation of why that's acceptable to players of JRPGs (and I'm an avid player of the genre), but that doesn't really fit into the game/narrative dichotomy that I've been discussing here, so I'll leave it out of this discussion.