XP bars are the most successful grinding mechanic.
The type I most like is resource grinding, it's also my most hatted if the developer only adds a item only to extend the grinding. Like how many RPGs have huge resource item list but only one character needs them to level.
The most fun grinding mechanic for me is the clue finding in Visual novels. Where often to fully solve the games you need to replay them to find all the missing parts.
In the end what makes grinding fun is the reward. If the reward wasn't worth the grind it will constantly upset players. Even games where grinding is worth the reward feels tiring when not balanced well.
33 minutes ago, Tom Sloper said:
To most players "grinding" means "repetitive actions the game makes me have to do.
This is absolutely true, even naming a fun mechanic "grinding" will turn it into a bad mechanic for players.
Grinding doesn't need to be repetitive, it just normally works out that way because of game loops. RPGs work by grinding monsters and to prevent it from being repetitive they change the way the monsters work.