Recently gave up on Diablo 2 nearing the end of the second chapter, inventory management became such a chore, after reviewing useless items for the 1xxxxxxxxxxxxth time I actually considered a job as a file archiver or at an assembly line to get more fun in my life. :^(
I kinda liked that one. :^)

You know so did I, for a while. It looked great and played fast, but at some point I thought "This is getting a tad repetitive!" :P Of course every hack & slash crawler suffers from that at some point.

Repetitiveness can be alleviated by adding the unforeseen.

An example: If you replay the Baldur's Gate intro in Candlekeep and the march over to the Friendly Arm Inn five times to find out which class feels best to you there will be little changes other than the way you play your character. Even loot is the same up to the amount of gold found in chests.
If you play a game like Morrowind you will find that some crates do have random loot. Even playing a cave for the 5th time will yield a different outcome. So if you play Baldur's Gate with the same character twice you will be pretty much at exactly the same spot, with exactly the same amount of gold in your pockets (unless you forget something), but in Morrowind you can start the game with exactly the same character, head into all the same dungeon, open all the same crates and two hours in you might be different levels with different weapons and different equipment.

I don't mean to say that Baldur's Gate is worse than Morrowind in any way, for me the opposite is true, but that Morrowind has a larger random factor in its gameplay which makes repetitive moments more fun.

There are lots of pretty repetitive, grindy games that I adore because unforeseen things happen even in repetition. The random encounters in Silent Storm take place on only a handful of maps, but each encounter can be tackled with a completely different tactic or can challenge you differently with opponents in randomized positions. In Mount & Blade Warband you will have to grind through one Searaider mob after the other to get your gold but each of those maps is different and even just small changes like hilly to flat terrain can make all the difference in tactics you need to apply. In Jagged Alliance 2 you will often have to fight the same map more than once (For example the crossroads that are usually occupied by Deidranna's forces) and you can change your experience drastically by just advancing in the game and once fighting that battle with pistols, once with SMGs and once with sniper rifle support.

In that regard, I wish more games would adopt the mini-game mentality that many of Ocean's film-licensed games in the late 80s/early 90s adopted (Robocop, Batman: The Movie, The Untouchables, Platoon, Total Recall). The games themselves might not have been very good, but you couldn't claim that they didn't have variety in the gameplay.

On a more general note, repetitiveness is fine if the game gives you the distinct feeling that you are contributing to achieving something. If all you're doing is moving to waypoints to advance the story and collecting useless items just for the sake of collecting them, that gets very old very quickly - game stories are rarely compelling enough to stand on their own, and the absence of any sense of emergent gameplay robs the game of any sense of agency.

But if the game gives you the chance to be easily diverted (GTA 4 did this superbly), or if those collectables serve a purpose to give you something more to do, that's when games come into their own.

Regarding the collectables, someone pointed out Assassin's Creed 1, which is indeed possibly the worst example of repetition out there. But the game starts out remarkably well, feeling very organic in structure, and the towers provide both excellent views and add something to the gameplay. It's just that it then devolves into storyline missions, the same random events, and those fucking flags.

I think the key to avoiding boredom is variation within repetition (same as in music). Right now, I'm playing Puzzle Dimension which is a very repetitive game but every 10 levels, a new gameplay element is introduced and that keeps things fresh enough.

What I don't like is when a game makes you repeat entire missions without any variation whatsoever. in Syndicate e.g. you have to do this a lot and it's to the detriment of an otherwise very exciting game. Another thing I've grown tired of are the random encounters in games like the Wizardry or Final Fantasy series. Too often, you'll fight a dozen or more random battles in a row - against the same 2-3 enemy types, requiring the same old approach and the only thing that changes is the number of enemies which is no real variation at all.
Randomly created content is a pitfall as well, as much as I loved Daggerfall and its unparalleled vastness, the recycled side quests and dungeons are going to get on your nerves eventually. Sometimes, entire dungeon segments were identical. Immersive atmosphere and a great soundtrack can compensate for a lot of repetition but there is a limit.

The most boringly repetitive game I've played lately was Seven Cities of Gold, there simply aren't enough gameplay elements and you keep doing the same few things over and over and none of them are even remotely exciting. Sailing from Spain to the New World and back is the most tedious part but trading, resupplying and exploring aren't interesting, either.

The thing that gets me is how repetitive modern games are. If content and variation = fun and replay value, then it would seem a 10-15 gig game would have a ton of content replay-ability, whereas as a much older, smaller size game would be limited. As a wise man once said, 'not so fast, my friend.' Quality usually does trump quantity.

Minecraft is also a good example. It is an absolutely amazing game, yet when you boil down its mechanics and function, you're essentially just rearranging numbers in a very simple geometric math equation. Then the game becomes mind destroying levels of boring and pointless. God sneezed out math onto a planet sphere, and your character has to run around the world, finding and reassembling the code. Your silly little redstone traps and infinite storage chests are just window dressing to the twilight zone hell you're actually in. And since you can reload and keep playing after dying, you really are stuck in a hellish universe that lasts for all of eternity... endlessly moving the same pixel blocks around for all time.

If you think about repetition for too long, it can drive you mad.