In reference to the December 20, 2011, Banned and Restricted Announcement.
After looking at the results of the Magic World Championships, the DCI is keeping the goal of not having top-tier decks that frequently win on turn three (or earlier). This was not an issue with either of the newly banned cards.
We also have the goal of maintaining a diverse format. While there were aggressive decks, control decks, attrition decks, and combination decks that succeeded, the diversity was not ideal. In particular, the heavy majority of all aggressive decks were "Zoo" decks. We looked at why other aggressive decks were not played, and after our analysis decided to ban two cards.
Punishing Fire, when combined with Grove of the Burnwillows, gives a repeatable 2 damage for 3 mana. This pair of cards is commonly used, and is devastating to creature decks relying on creatures with less than 2 toughness. It also is a very slow and reliable win condition, netting 1 life for 3 mana. Tribal decks relying on 2 toughness "lords" see very little play, and this is a major barrier to their success.
Wild Nacatl is a creature that simply attacks and blocks very efficiently. It is very unusual to ban such a card. We looked at our Modern tournaments and previous Extended tournaments to find when the attacking decks were fairly diverse, and when they were dominated by Zoo. At Pro Tour Austin 2009, won by Brian Kibler playing Zoo, most attack decks were of the Zoo variety. Next year, at Pro Tour Amsterdam 2010, won by Paul Rietzl playing white weenie, the Top 8 had a variety of attacking decks: Brad Nelson and Brian Kibler played Doran decks, Paul Rietzl and Kai Budde played White Weenie, and Marijn Lybaert played Merfolk. The format was different but, other than the lands, the only cards in Brian's main deck that could not be played were two copies of Lightning Helix. The lands were the big change, and a very important difference was that the mana base in Austin meant Wild Nacatl was a very reliable 3/3, but would not be so reliable in Amsterdam.
We looked for cards to unban, but not only could you play the Amsterdam deck as is, other powerful cards are already available in Modern. For example, Æther Vial was unavailable to Marijn, but is legal in Modern. The Vial is considered one of the stronger cards in Legacy Merfolk decks. The problem is that other decks try to use synergy to get rewards, but those rewards aren't any better than the Wild Nacatl. For example, the Doran decks use Treefolk Harbinger to find Doran. When it all works, the Harbinger is effectively a 3/3 for . With shock lands, Wild Nacatl is a 3/3, and doesn't let you down when your opponent kills your Doran. With some effort, Student of Warfare becomes a 3/3 First Strike creature, but that isn't a sufficient reward for the effort compared with Wild Nacatl. This creature is so efficient it is keeping too many other creature decks from being competitive. So, in the interest of diversity, the DCI is banning Wild Nacatl.
Modern now has 29 banned cards out of an available pool of 6,535 cards. That is a lot of cards that have been banned in a matter of months, but I do not expect that rate to continue. The format is closing in on our initial vision in terms of speed and metagame diversity.
A more subtle reason for the banning of Wild Nacatl is the development of future Magic sets.
The last time we started a new banned list for a non-rotating format, it was for Legacy (then called "Type 1.5"). Legacy used to use the same list as Vintage, and the decision was made for it to have its own banned list, which had 62 cards banned out of an available pool of 6,449 cards. That is a slightly smaller pool, and more than double the number of banned cards. Many of the banned cards were made before there was a notion of large competitive tournaments. Others were made before Legacy had its own banned list. Now, cards are added to the banned list at a much slower rate—maybe a card a two a year. Why is that? It is because R&D is aware of the format. Most of the cards that would result in bannings aren't printed. But Magic is complicated and a few cards are still printed that result in bannings. However, the rate seems to have fallen to about a card or two per year. This year, Mental Misstep was banned in Legacy. In Vintage, no cards were banned and Fact or Fiction is no longer restricted.
Modern has cards that are banned that would likely not be printed now that development is aware of the format. When Glimmerpost was put in Scars of Mirrodin, R&D was aware it would be powerful with Cloudpost. Developers were thinking the combination was safe for Legacy, unaware that soon there would be a Modern format where this could be too powerful.
When Wild Nacatl was tested, developers recognized it would be powerful in Legacy, and its power in Extended would vary as lands rotated in and out of the format. Developers didn't consider the non-rotating Modern format, which did not exist at the time. It looks very strange banning a creature that only attacks and blocks. Generally, combat creatures don't get banned. The reason isn't that the power level of combat creatures inherently isn't high enough to ban. Rather, it is because R&D tends to have a reasonable idea of how attacking decks work and has an idea of how strong each creature is. If a designer made a 1-mana 5/5 creature, development would know the card is too good and it would not see print. With a card like, say, Hypergenesis, it is harder to know what will happen.
We have high hopes for the Modern format, and its upcoming PTQ season.