What sort of mechanical changes would cause you to quit?

MMOs often undergo extensive mechanical changes throughout their lifespan, and while sometimes said changes do serve the larger game and its future profitability, other changes seem destined to piss off current players in service of chasing a new potential player group.

If you've played in one particular MMO long enough, chances are you've lived through significant game changes, whether they're basic class revamps or more severe alterations like combat system makeovers or the removal of core mechanics like item decay. Assuming you haven't quit your MMO of choice over extensive mechanical changes, what kind of tweaks could prompt you to do so?

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