So just to understand this a bit more, mythics are typically cards with powerful abilities that are not found anywhere else? And goyf's power is generally in line with that of rares?
Eh, not really. What differentiates a normal rare and a mythic rare is:
1. "Splashiness". That is to say, mythic rare cards should ideally make you go "wow" as soon as you see them, because they
look powerful or
look really unique. In actuality, there is rarely much difference between the average rare power and the average mythic rare power of a set.
2. Planeswalkers are only printed at mythic rare now.
3. For the most part, legendary creatures are now printed at mythic rare. If there are a ton in a set, like in Return to Ravnica block and Theros block, then some "less important" characters will be at rare.
4. Lands that are expected to appear in a lot of decks, like dual color lands, never appear at mythic rare. Lands like Maze's End are fine at mythic rare.
5. Sometimes, cards are placed in mythic rare if even just having them at rare will warp the limited environment. I can't think of any actual examples, but I know people were saying they should have done this with Pack Rat back when RtR was in limited.
In the end, it's mainly a feel thing. Pack Rat doesn't feel like it would be appropriate as a mythic rare, for instance, and while Kruphix, God of Horizons sees no tournament play, few would think he should have been a rare instead of a mythic.