Harpie's Feather Duster
Spell Card
SDD-003, Secret Rare (Yu-Gi-Oh! Worldwide Edition: Stairway to the Destined Duel Promo)

"Destroys all of your opponent's Magic and Trap Cards on the field."

Card Review
Harpie's Feather Duster is the ultimate spell and trap removal card that has ever existed. It wipes the opponent's Spell/Trap field clean at more or less zero cost, removing all hidden threats and blowing up almost any kind of lock. It helps cripple Spell/Trap decks, FINAL decks, Exodia decks, burn decks, Spirit decks, and anything else that requires the extended presence of continuous cards. A perfect 10/10 for Phenominal Cosmic Power, but 0/10 for what it does to the metagame.

Why the Card is Banned
I pretty much explained this above, but to reiterate, this card is far too powerful and makes any extended strategy involving continuous Spells and Traps difficult to pull off. Of course, Harpie's Feather Duster was often wasted via over-extension simply to blow up 2 or 3 unimportant cards, but when used smartly, it won duels. This card is banned because it is in the class of costless mass destruction cards.

Best Replacements
There are many cards which can be used as a replacement for Harpie's Feather Duster. The current metagame favorite seems to be Heavy Storm, which is the Dark Hole equivalent for Spells and Traps. Heavy Storm destroys all Spell and Trap cards on the field, including your own. Despite its power, it is rarely run in pre-ban simply because Harpie's Feather Duster is sufficient on its own. It is possible that Heavy Storm could also be banned in the future, however. Giant Trunade can accomplish the same thing for a single turn, but it only returns cards to the users' hands, and thus is not considered a worthy replacement. If you don't have any of the remaining mass Spell/Trap removal cards or you don't feel you need them in post-ban, at least replace this with a card that can remove or negate some limited number of Spells and Traps. Dust Tornado, Magic Jammer, and Judgment of Anubis are examples, although the first is already being used to replace a Mystical Space Typhoon in many decks, as MST is now limited to one.

My Opinion of the Ban
You can't really have a card more worthy of being banned. I love my Harpie's Feather Duster and would never trade it, but even I am glad to see it banned in "Advanced" format. When playing under this format, I can never quite get accustomed to not fearing it. I still find myself thinking about whether or not my opponent has played his or her Feather Duster yet, and then I remember it is banned. When cards cause so much gameplay anxiety that you must make special note of whether they've been played or not in every single duel, that's a good indication that they are horribly broken.

Probability of Unbanning
If this card is ever unbanned, then I'll no longer be sure of the ban list's purpose. Don't expect this to ever come off or another card like it to ever be created, but all the same, don't get rid of your Feather Duster. There are still pre-ban tournaments and casual play where it can be used, and as I've stated in my newest rant, we need to support these formats just as much as "Advanced" tournament play, or this game is going to be irreparably damaged. I for one intend to keep myself sharp in both formats.