Enchantments are suppost to be revealled and benefit you immediatly.
I thought this was an interesting point, but wasn't sure of its umm integration with the MW design ethos/intent. The end of this 'benefit you immediately' seems more a true statement for incantations. I mean if you cast and incantation and it did nothing, itd be a dumb card (or really poor play). But an enchantment is played face down for tempo, bluff, hedging your bets, mana conservation etc. Knowing/deciding when to reveal it is a big part of the ebb and flow of the game, too soon you reveal your hand (sic), too late and you miss the optimum timing.
Like someone (Sike I think) said, AF seems like it could go either way and I can see merits to both. But it doesn't seem that big a deal for the card to get revealed before the dice roll. You've got something you really need to make sure you kill, you reveal it then. Having it revealed doesn't negate the card or make it useless. In some ways saving it up until you know it must be used seems a bit stronger, for 2 mana you can guarantee one timely re-roll (still doesnt guarantee a good roll, but its a statistical improvement).
I guess one of the problems is that the dice roll is the primary point of uncertainty. So almost everything else you can better estimate when you should reveal for best initial/immediate benefit. AF revealed before a dice roll doesnt give you that certainty, you might reveal it, then roll awesomely. Does that mean it wasn't worth having revealed it? If your creature gets killed that round, probably annoying.... otherwise its an emerging threat to be considered.
I can't think of another decision with the same uncertainty - the other main uncertainty is around opponent face down enchantments, or what they will do with their actions. But all of these would give you opportunity to reveal your own enchantments before resolution i.e. if a creature with a large stack of hidden enchantments declared an attack on your imp with face down rhino hide or similar, you can wait right through to the end of step 5 before you reveal it, so you'll have seen whatever they've rolled and what effects they're using before you decide. I have been caught out by surprise teleports or rouse the beast moves which have meant I thought I'd picked the right moment to reveal my protection/use my defense and something nastier came along, but that's good play (or bad on my part).
I guess Jack's comment seems as good a question of intent as any, just how good an in the pocket card was AF meant to be?
PS I don't mean to be rule lawyering the fun out of the game, I was just enjoying the curves of the discussion. There are plenty of other threads that just seem meh, this one seems interesting and has people bouncing some interesting design/intent/solution thoughts around.