Believable

Believable: a word used to describe something that can be easily accepted as truth, even by the skeptic mind.

Given that the Agents are a suspicious lot, this doesn't encompass a whole lot.

Goodfic tends to be believable given the limitations of its source canon. Badfic, on the other hand, is often not.

For example:

Severus Snape shagging Remus Lupin is not believable.

Aragorn as a mysogynist bastard is not believable.

Sesshomaru falling in love with a random female instantly is not believable.

On the other hand, Glorfindel slaying a Balrog is perfectly believable, as it has happened in canon.

Same goes for Harry Potter slaying a full-grown basilisk at age twelve, albeit with some help.

Seeing a pattern here? Let's hope so.