I think the general principle is that the protagonist shouldn't have more than maybe one important advantage over most other characters. If most of the characters in the fic are non-morons, or superpowered, then that one's free. Luminous!Bella gets to be a vampire "for free" because she mostly interacts with other vampires; Rational!Harry gets to be rational because everyone else in MoR is smart too; but for Bella to be both luminous and a witch, without most of the other main characters in the fic being one or the other as well, wants a single cause.
Alice and Edward are the most significant Cullens, and have witchy powers. The antagonists include a large proportion of witches. Does this get Bella a witchcraft pass? Particularly since her power is one that only works in the context of other powers - it would simply not do anything in the absence of other witches.
In the vein of the Harry Potter and the Methods of Rationality discussion threads this is the place to discuss anything relating to Alicorn's Twilight fanfic Luminosity. The fanfic is also archived on Alicorn's own website <strike>(warning: white text on black background)</strike>.
Previous discussion is hidden so deeply within the first Methods of Rationality thread that it's difficult to find even if you already know it exists.
Similar to how Eliezer's fanfic popularizes material from his sequences Alicorn is using the insights from her Luminosity sequence.
Spoilers for the fanfic itself as well as the original novels need and should not be hidden, but spoiler protection still applies for any other works of fiction, except for Harry Potter and the Methods of Rationality chapters more than a week old so we can freely discuss similarities and differences.
EDIT: Post-ginormous-spoiler discussion should go to the second thread. (If you have any doubt on whether you have reached the spoiler in question you have not.)