Bestow is a new keyword from Theros. It can be only on Enchantment Creatures and it has the following reminder text:
If you cast this card for its bestow cost, it’s an Aura spell with enchant creature. It becomes a creature again if it’s not attached to a creature.
Now, what happens if you cast it with bestow and before resolving, the targeted creature is destroyed? The reminder text is not explicit, but MaRo says, the following about this (paraphrasing): long-time players will probably read the ruling, newer players will be optimistic in interpreting this as they want it to work.
Since our Commander games often include some younger players and I sometimes play newer players in FNMs (well, everyone’s younger than I am), I know this to be true: if there’s room for interpretation, they will try to interpret it in their own favor. Older players know games like to screw players and make things harder just for the sake of leaving no room for exploits, but this often means exceptions and exceptions to exceptions. Complexity of rules is not good when you’re trying to reach a mass audience, such as the around 12 million people who play Magic, although the game itself can still remain quite complex.
I think this is a good approach from Wizards. Granted, their resources are on a whole different level from most games, as they have fully dedicated teams of people, working on sets for many, many years, so they have the opportunity to put an effort on this sort of thing. On the other hand, the instinct to be the cruel master when designing the game, is often very strong, but should be avoided. The point is to draw people in with good experiences.