While my coverage compatriots Rashad Miller, Brian David-Marshall, and Nick Fang were cranking out the numbers behind the Draft metagame breakdown, I took some time to research what exactly defined the archetypes. Using my own experiences in developing the set and playing it upon release as well as talking to Brian Kibler, Brad Nelson, Kyle Boggemes, Jan Doise, Christophe Gregoir, and Peter Vieren, here is how Scars of Mirrodin Draft breaks down into archetypes.
First, be aware that the individual color decks and color-pair decks aren't defined here. Those are mono-red decks or white-blue decks. You can put together a successful draft strategy simply sticking to draft basics by picking an aggressive curve with some removal spells and not paying much heed to the intricacies of the greater Scars format at large. Ignoring metalcraft, infect, and a number of other options can still be successful if you find yourself unable to come up with a more coherent strategy. Just remember to value evasion creatures higher than non-evasion creatures, that a Runeclaw Bear is powerful no matter what the haters say, and that removal still trumps random dudes.
Now for the more specific archetypes...
This is the big one, the gorilla in the room. Infect-based decks have been called by many "the most powerful in the format." That has been borne out in part by results at some of the premier-level events featuring the 40-card Scars format (though Gerry Thompson managed to win Grand Prix–Nashville without relying on the strategy). The most common builds in the archetype feature black and green cards, but other color combinations can certainly be supported if you simply draft enough cards with infect. The "rules" of the infect decks:
1. According to none other than Brad Nelson, Cystbearer is THE most important green common for the deck. Its butt of 3 makes it more powerful than the Gray Ogre that serves as the "vanilla average" for the three-drop slot. That also makes it immune to some of the common (as in frequent, not referring to rarity) burn spells like Galvanic Blast and Arc Trail.
2. The key aspect of infect decks, whatever colors you play, is to value two-drops highly. Ichorclaw Myr and Plague Stinger are your best bets here, but Necropede serves as a reasonable alternative if you can't find yourself any of the others in your packs.
3. Corpse Cur is a marked upgrade for Gravedigger loops, and is a powerful, key addition to infect decks. In previous Limited formats a Gravedigger loop in which you cast a Gravedigger to get a creature back from your graveyard, then block with the 'Digger and cast a second to regrow the first, perpetually blocking your opponent's biggest creatures, has always been powerful. In Scars of Mirrodin, however, your "Gravediggers" (Corpse Curs) actually grind down your opponent's creatures with -1/-1 counters, making attacking into the loop a much trickier option. The 2/2 is also superb recurring things like Ichor Rats or Putrefax.
5. Finally, and this came directly from Brad, mixes of black, green, and red as infect creatures backed by red's removal to force through extra poisonous damage can be VERY powerful.
If Infect decks are the gorilla in the room, Metalcraft strategies might be "the next-largest great ape" in the room of Scars of Mirrodin Draft (yep, that's right). They are popular because, unlike infect decks, you have dozens of cards you can use to power them. You don't need to look for the word "infect" on the cards you add to your deck, but simply "artifact." Being on the plane of Mirrodin makes that a pretty easy proposition to come up with.
Red-white metalcraft is arguably the most popular of these archetypes, but it is not the only color combination needed to consider yourself a metalcraft mage. The key to the archetype is valuing mana Myr highly (Silver Myr, Iron Myr, etc.) and looking for cards like Origin Spellbomb. Those cards will power Rusted Relics, Chrome Steeds, Snapsail Gliders, Vedalken Certarchs, and more.
So how many artifacts make a metalcraft deck? Brian Kibler swears by a minimum of 14, but prefers 17 or more. With nearly half your deck dedicated to artifacts and about half dedicated to land, you have precious few slots left over for colored spells. So which ones should you be playing? Certainly metalcraft cards like Blade-Tribe Berserkers and Vedalken Certarch are good inclusions, though one has to be careful to remember they don't count towards your metalcraft count. Mostly, however, those slots will be the removal spells you can get your hands on, cards like Galvanic Blast (particularly good in metalcraft) and Arrest.
Perhaps the most important lesson of playing a metalcraft deck: PLAY ARTIFACTS! The archetype's quirky need of the metal permanents has led to cards that might never make the cut in other formats regularly being sought after by the world's best players.
A strategy popularized by Conley Woods, Furnace Celebration decks center around ... well, Furnace Celebration. They try to find ways to sacrifice their own permanents in order to trigger the Celebration and ping their opponent to death (or in a pinch their creatures). In general you aren't going to go into a draft expecting to draft this archetype, but if your first pack shows two of the uncommon going around late, you can pick them up and switch gears pretty quickly.
The key cards to the deck are Spellbombs. They're the easiest artifacts to sacrifice, but more importantly they provide a cantrip for doing so. That's key, because without the actual Celebration on the battlefield you can find yourself playing a bad "do nothing" deck while waiting for it to make an appearance. In those situations, all you'll want to do is draw cards until you find Celebration.
To help you sacrifice other cards you want a tag team of Ferrovore and Barrage Ogre. Both can function on their own, and Barrage Ogre is particularly popular already. Culling Dais, Oxidda Daredevil, and Dross Hopper can work in a pinch, but they are the B teamers in the Furnace Celebration lineup. Overall the archetype is very powerful, but extremely high risk. When it wins, it wins big, but when it doesn't? Well, it's best not to think of that ....
Ari Lax helped to popularize the Dinosaurs archetype in the finals of Grand Prix–Nashville. The deck is so named from the fact its key creatures, Molder Beast, Alpha Tyrranax, and Bellowing Tanglewurm, happen to look loosely like dinosaurs (more like terrible lizards, but let's not quibble ...). So how do you draft them?
Humorously, as a Belgian contingent of pros including Christophe Gregoir, Jan Doise, and Peter Vieren (who had just drafted it in the second pod) informed me, you draft Dinosaurs by NOT drafting them. Rather, the strength of the strategy is that you get the big green creatures late in a sign from the drafters around you that they don't actually want them. That means your early picks are actually spent on removal spells. Slice in Twain is a heavens-sent gift, but playing green means you can grab the most powerful spells from basically any color. Got some Arrests? You're green-white! Picked up a Skinrender and Grasp of Darkness? You're black-green! Picked up Arc Trail AND Arrest? Grab some Horizon Spellbombs and off-color Myr and you're three colors!
The Dinosaurs archetype is great for punishing a table that gets too greedy with metalcraft and infect strategies to bother drafting "dumb" green creatures and get there in the Red Zone. You can punish them for their reticence and play the coolest named deck while doing so!