The recent rules change added an additional check for the legality of casting a spell. This check occurs before you begin to cast the spell:

Originally posted by 601.2:

A player must be legally allowed to cast the spell to begin this process, ignoring any effect that would prohibit that spell from being cast based on information determined during the spell's proposal.

Grid Monitor

Under Grid Monitor, can I cast a creature spell for its bestow cost? Grid Monitor says it is illegal to cast creature spells. At the moment of the 601.2 check, it is a creature spell (it's definitely a creature, the rules seem to refer to it as a spell at this time even though it isn't on the stack yet). Grid Monitor's effect would not prohibit the spell from being cast based on information that will be determined during the spell's proposal, it prohibits it from being cast based on it's characteristics right now.

If you control a Grid Monitor, you can cast bestow cards such as Hopeful Eidolon for their bestow costs, because that is not casting a creature spell–that is casting a noncreature Aura enchantment spell. You cannot cast them for their normal mana cost, because that would be casting a creature spell.

The legality check outlined in CR601.2 is referring to whether or not the player has permission, provided either by the rules or by some other effect, to cast the card in question in the first place, regardless of any of its characteristics. For example, can I cast one of the cards from my opponent's hand? No, because nothing gives me permission to cast cards from my opponent's hand.

Effects that prevent you from casting spells with particular characteristics, such as Grid Monitor, Reflector Mage, and Void Winnower, are not covered by that clause. Instead, these effects are covered by the legality check outlined in CR601.2e, after the proposal of the spell has been completed.