After establishing that Affinity was the obvious deck for this Block format, the next goal of most play test groups was to find the obvious countermeasure. How do you beat a deck full of artifacts? Well, you build a deck that destroys them.
Inevitably, you turn to Red and Green. The options there are almost limitless: Detonate, Viridian Shaman, Shatter, Oxidize, Glissa Sunseeker, Molder Slug, Echoing Ruin, Dismantle. The only problem is, when you load your deck with too much artifact removal, you suddenly have a lot of dead cards in say, the mirror match. With this in mind, most of the Green-Red decks are running Viridian Shaman, Molder Slug and perhaps Glissa, since none of them are dead cards against decks without many artifacts. Oxidize is the other maindeck spells of choice due to its sheer efficiency. One of the best ways to attack Affinity decks is to hit their lands, and Oxidizing an artifact land on turn one instead of having to wait for more expensive removal can be the key to winning that matchup.
The other question is the mana acceleration. Green/Red players in the past have always had Birds of Paradise, Llanowar Elves, or even Wirewood Elf as their early drops. This time, though, the options are mainly Talisman of Impulse and the Myrs. While none of them sound terrible, players quickly realized that one of the huge advantages of playing this deck was that it made all of your opponent's artifact removal into dead cards, and putting a Talisman on the board just counteracts that. Suddenly, Darksteel Ingot has become the Green/Red mana acceleration of choice. It still allows turn four Arc-Sloggers and Molder Slugs but leaves your opponent's Oxidizes as the dead cards that you want them to be.
After that, it's a matter of filling out the rest of the deck. Troll Ascetic has been seeing a fair amount of play, which isn't surprising since it's arguably the best Green creature in many years. Fireball has also been enjoying a comeback as a finisher that doubles as removal for opposing Arc-Sloggers and other men. Electrostatic Bolt and Grab the Reins also often make the cut as additional removal. A nice side effect of Grab the Reins is that it's an answer to one of the surprise decks of the tournament, mono-Green Tooth and Nail. There's no better feeling than stealing a Darksteel Colossus and tossing it at your opponent's head, although if they anticipate this they'll probably search up Leonin Abunas as well.
Quite a few players also decided to give up on the Green altogether. The Italians have a mono-Red concoction that plays Solemn Simulacrum as its only artifact. Slith Firewalker provides early beats while Arc-Slogger brings up the rear. The deck uses Detonate and Echoing Ruin to put the hurt on the lands of Affinity decks and even includes Molten Rain for additional land destruction.
Kai Budde and the Germans have taken a different approach to the mono-Red deck. They've traded in the concept of an artifact-less build for the sheer power of Furnace Dragon. Great Furnaces, Talismans, Cloudposts and Chromatic Spheres allow them to accelerate out a Dragon quickly enough to stop Affinity before it gets out of control. Even when the ability isn't devastating, the 5/5 flying body can be enough to bring an end to many games, and the deck is built so that none of its own threats die to the entering play effect.
All of these decks have the ability to burn players out from large life totals, and it's not uncommon to see Pro Tour matches ended by nine point fireballs. Some players are even trying out Pulse of the Forge as an additional way to end games directly. In any case, these decks are packing destruction, mayhem, large creatures, and lots of one of mankind's oldest weapons: fire. It doesn't get any better than that.