<p>Do you primarily think of reasons TO implement it, or reasons NOT TO implement it? What are the advantages of each?</p>

I think first about the stakeholder (the client).
Will that feature help
him
? Is it indeed a functional feature that brings value?
Then I think about technical implications and the resulting complexity, in order to evaluate the trade off between implementation cost vs. not having that feature.
Based on those two first elements of reflexions, I can begin to know if I must implement it or not.
I guess there is no clear cut between reasons to or not not implement it.