Vote Down requires 125 reputation
Shouldn't it be "reputations"? Why or why not?
Reputation in this instance is being used as a mass noun, and mass nouns do not normally take a plural. It is somewhat unusual in this case that there is no count noun to serve as the measure word, i.e. we don't say "125 points of reputation" by analogy with "125 grains of rice". Nonetheless, the formulation given above is what is normal and idiomatic for this scenario.
There are other words which exhibit this same pattern:
All of the examples that come to mind here are related to technology or gaming. I don't know if this is an actual trend, or just selection bias based on what's in my brain.
Reputation is not the unit, its the object being quantified. Technically, it would be
Vote Down requires 125 points of reputation
But this is awkward and we all understand that reputation is an arbitrary number with no real unit. Saying "How much reputations do you have" would be akin to saying "how much strengths do you have" or "how much sauces do you have?"
Well it looks like you can say reputations :
The book investigates how reputations historically have been made and un-made (from a study 'Surveillance & Society' by Daniel Neyland Oxford University)