Bestow looks like a rules headache. I think the intent is that if you kill the creature underneath the "aura," the "aura" becomes a creature again (so it's like a better Griffin Guide). But what happens if you kill the creature in response? I assume since the card is cast for Bestow, it targets, so it will still get countered and will go to the graveyard. I assume if you cast it for Bestow, it loses Creature and gains Aura?
That being said, Bestow looks like the most interesting mechanic in terms of design space.
The Monstrous mechanic looks like it will stay a Timmy mechanic, nothing more.
Heroic is a clever way to get some extra value out of Auras.
Devotion is clearly their attempt to reward mono-colored decks. It's another mechanic that's "meh" on its own, and will come down entirely to how it's implemented.
EDIT: Kudos to them, by the way, for figuring out how to make a creature meaningfully an enchantment and a creature at the same time.