Only three guilds have shown up in any significant numbers without any third color support. These decks take a lot of strain off of teammates' decks by not stealing any significant number of spells and almost never stealing lands they might need.
Orzhov: Black-white decks have made a strong showing in Standard, and that success has transferred to Team Block as well. The exact composition of the decks varies greatly, from quick aggro-discard combinations to plodding Hour of Reckoning concoctions, but they all pack versatile removal and hard-to-answer threats.
Key cards: Mortify, Ghost Council of Orzhova
Rakdos: Discard, burn, removal, card advantage - these are the ingredients of the best Rakdos decks, all off which try to win the game quickly by tripping opponents up before they can set up their mana. This is the black deck with the lowest overall mana curve in the format, and is the best place to stick Dark Confidant.
Key cards: Dark Confidant; Lyzolda the Blood Witch; Rakdos Guildmage; Crypt Champion
Simic: With efficient creatures up and down the curve, plus access to countermagic, combat tricks, and an odd form of removal in Cytoshape, graft-based Simic decks can put the pressure on opponents like no other guild.
Key cards: Vinelasher Kudzu, Moldervine Cloak, Simic Guildmage, Plaxcaster Frogling, Cytoplast Root-kin
This isn't to say that there aren't straight Boros or Gruul decks in the event, it's just that their aren't very many of them. Most guilds in the block need some help plugging holes in this format. The bulk of Friday's decks are of the three-color variety, combining the best of three guilds into one saucy package.
Black-white-green: The most popular execution of this combination is akin to the "Beach House" deck from Pro Tour-Honolulu, later known as "Roxodon Hierarchy." The deck plays for the long-game mana advantage, figuring that the first player to get Angel of Despair or Skeletal Vampire to stick will win.
Key cards: Loxodon Hierarch, Angel of Despair, Skeletal Vampire, Rolling Spoil, Crime//Punishment
Green-white-blue: The Supply//Demand-based Dovescape deck that sprung up at Regionals this year transitions more-or-less seamlessly to Team Block. All the usual suspects are here, of course - Grand Arbiter, Simic Sky Swallower, etc. - and they're usually joined by a solid core of workhorses like Watchwolf and Court Hussar.
Key cards: Supply//Demand, Grand Arbiter Augustin IV, Glare of Subdual, Loxodon Hierarch, Simic Sky Swallower
White-blue-black: Another deck that takes advantage of the Arbiter, although this deck is far less creature-based than the green versions. Instead, this deck uses the Arbiter to power out card drawing and counterspells, eventually winning with Orzhov finishers.
Key cards: Grand Arbiter Augustin IV, Mortify, Compulsive Research, Angel of Despair
Red-white-blue: There are two flavors of this deck - one with Searing Meditation and one without. Both pack lots of Firemane Angels for the long game and plenty of life gain and card drawing to get there.
Key cards: Firemane Angel, Electrolyze, Faith's Fetters, Lightning Helix, Odds/Ends, Compulsive Research, Searing Meditation
Red-green-blue: Most versions of these decks look like Izzet-based control decks with access to green's mana acceleration and the board-sweeping power of Savage Twister. High-toughness creatures are the order of the day, from Carven Caryatid to Rumbling Slum to Simic Sky Swallower.
Key cards: Savage Twister, Simic Sky Swallower, Carven Caryatid, Electrolyze, Compulsive Research, Demonfire
Black-blue-green: This trio is the most schizophrenic, with some players opting for Birds of Paradise and Elves of Deep Shadow to fuel beatdown (usually with Dark Confidant), with others going for a controlling strategy using the removal and counterspells from the Dimir, Golgari, and Simic.
Key cards: Birds of Paradise, Dark Confidant, Trygon Predator, Plaxcaster FroglingOR Voidslime, Putrefy, Simic Sky Swallower, Skeletal Vampire, Twisted Justice
The other three-color combinations (GRB, UBR, WGR) do not appear to be major players in this field, although I'm sure there will be some surprises when we comb through the decklists on Day 2.
The Greediest: Four- and Five-Color Decks
Even with the requirement that you can't have more than four copies of a single card between all three decks, some teams have opted to let one player "have it all," and play a four-plus color deck. Often this means that his teammates have to "donate" one of their dual lands so that it can be fetched out with Farseek, but the teams that do it agree that having an "everything" deck is the best way to use every good card in the format.
Four-plus color control: These decks involve every spare removal spell, counterspell, and Wrath, combined with a healthy mix of mana fixing and finishers.
Key cards: Farseek, Simic Sky Swallower, Demonfire
Four-plus color land destruction: I've seen just about every bad "Stone Rain" variant in the block played to some effect so far Friday, and this deck combines as many as it can into one deck.
Key cards: Farseek, Rolling Spoil, Wrecking Ball, Wreak Havoc
Pillar Beatdown: Take every gold aggro card in the format, toss in 4x Pillar of the Paruns, and shake. I saw Wessel Oomens play cards from five different guilds in the first six turns of one game Friday.
Key cards: Pillar of the Paruns, Watchwolf, Burning-Tree Shaman, Lightning Helix, Skyknight Legionnaire, Blind Hunter
This list doesn't begin to cover the big puzzle - fitting three decks together under the Unified Constructed rules. What cards can one deck afford to give up? What cards can be split across multiple decks? Those that could answer all those questions will be playing Saturday. Stay tuned.