"Endless Grinding" was my vote, but I would like to say that's under a fairly specific context; as in literally "endless", not just figuratively.
To be more precise, the type of endless grinding which the MMORPG genre implements; the grind is continuously being extended through the addition of more content, which is then encouraged by having that new content requiring that you acquire the new gear from said content in order to complete it. Ultimately, the point of the game then devolves into a functionally unending gear-progression-ladder ("GPL" for short); one frequently subject to absurd stat-inflation and often encourages some of the worst cases of elitism you'll find in any gaming community.
To say these games drag their content out is an understatement, as they frequently are played for years by the same people; almost everyone will eventually end up quitting the game due to them being fed up with this endless cycle, typically well past the point the game has over-stayed its welcome. The inherent addictiveness of the genre isn't helping either.
As sad as it is to say, someone has contrived a way to make a game functionally endless... and people have actually fallen for it en masse (and are paying a periodic subscription fee to keep playing it). The game is constantly being lengthened, and seldom are those extensions actually beneficial to the game.
And yes, I do have a bit of a grudge against World of Warcraft. Why do you ask?
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As for the other options in the poll: back-tracking, fetch-quests, and cutscenes? Those are at least enjoyable to a degree, if not by everyone. Back-tracking is an integral part of games where exploration is a strong element, with the player finding new routes and short-cuts through older areas as their capabilities increase; finding those, along with other hidden secrets, can give back-tracking an appeal to many. Fetch-quests, while they tend to be boring and/or redundant on their own, can lead to other discoveries and diversions; there's also the possibility that a fetch-quest could be executed quite well with a good premise, interesting story elements, a neat setting, unique mechanics, and so on (though you'd probably forget it is fundamentally a fetch-quest at that point). And cutscenes? A good story can be conveyed through those cut-scenes, and good pacing won't make the fact they're unskippable a problem (repeat play-throughs not withstanding).
But endless grinding? Never is that a good thing, especially when it's quite literal. At best, grinding is integrated with other gameplay (and/or story) elements; such that the player never feels like they're actually grinding.
The moment the player feels like they're grinding, you have a problem.