Whenever I post on grinding, there are the inevitable comments from people who enjoy it. The note that seems to be missing is, “in some ways, in some contexts, at my option.”
Good grind gives you the choice in those things. Having played MMOs for years, I clearly must love me some mindless repetition for its own sake, and sometimes that is really what you want. Killing ten rats is reliable in its difficulty and reward, comforting in its consistency. There really are times when you just want to farm something. It lets you decompress after work, it gives you something to do while chatting, or it lets you gear up to or down from more intense activity.
There are many examples of good grind. The game might not kick you out after you beat it, so you can keep playing if you like. “Survival mode” is popular, in which you click the button for an endless stream of zombies. I love New Game Plus in all its forms. (My apologies again to Kingdom of Loathing for whining that it was feeling grindy on my 80th Ascension or so.) Or maybe you can do a bit of rat-killing on the side for a cosmetic reward or a small, non-essential buff.
Bad grind is required. Bad grind is when the NPC says, “Now go do that five more times!” before you get any new options. Bad grind is when no NPC says that but the next boss is balanced around your having farmed five more levels. Bad grind is when a game has 8 hours of content and makes you repeat it 5 times so that it feels like 40 hours of content before the big end scene. Bad is when “New Game Plus” is required to get to the “real” game (arguably: or all the trophies/badges/achievements). Bad grind is when the repetition is not the game itself but is keeping you from the rest of the game.
There is an intermediate case where the grind is the entire point. (Some might say that MMOs inherently fall into this category.) You know walking in that you are going to be doing the same thing for hours. That is why you are there. Either you classify the whole thing as “not fun” and skip it, or you wallow in it. The only problem is those poor souls who wandered in expecting something else.