I think most RPGs are a great candidate for this question. If a villain suddenly knows about leveling and gear stats, his main goal would become to level up as much as possible, potentially starting a whole genocide to farm XP
Also how would politics and society change? Do you tax level ups? Make new laws that keep certain builds illegal? Would you be able to join the Imperial Army after completing the Dark Brotherhood in Skyrim?
Not related to the question but do you know how much positive karma it took before you could post here on R/Gaming it keeps telling me I need positive karma