This is a sort of a repeatable Prison Term, but not really. It easily locks down any unshrouded creatures until someone casts a creature spell. This is a great card for a control deck since you don't need that many creatures and this is just a great control card. The fact that it turns creatures into the marginally more vulnerable type of enchantment. It allows for an Azorius control deck to run more than a couple true kill spells because it turns cards like Erase into a better Path to Exile. Yeah, there's a flaw in that its effect is undone by a creature spell, but it can be worked around by timing it just right. That Blightsteel Colossus won't do much damage during the combat step if it's an enchantment now will it?
Pros: Makes creatures into enchantments, makes creatures lose abilities
Cons: Undone by creature spells