You might go into a draft looking to stay open for as long as possible, then commit to an archetype within the first few picks because you opened, and got passed, a few cards that are really worth building around. Or you might go into a draft looking to force a specific archetype (if you get an opportunity to do so), but ultimately switching into a wait-and-see role when you don't get passed any good cards for the deck in the first few picks.
If you're drafting a themeless good stuff deck (the no-theme theme) you can wait for a long time before you commit to a second color. But in order to put together (most) theme decks, you will generally need to start picking up enablers pretty early.
What's in a Theme?
It's generally pretty clear when someone is drafting a theme in New Phyrexia / Mirrodin Besieged / Scars of Mirrodin Booster draft. If someone is drafting infect, they're going to have a ton of infect creatures. If a player is putting together a metalcraft deck, he or she is going to need to grab a whole bunch of artifacts.
You will often end up drafting, and playing, some mediocre artifacts to help fill out your metalcraft deck. And even if you don't have to play any chaff, you're still going to up your valuation of things like Flayer Husks, Spellbombs, and mana Myr relative to how you would value them if you weren't concerned about your artifact count.
If you don't prioritize drafting artifacts over cards that are marginally better in isolation, then your "dedicated" metalcraft deck will often get stuck on two or fewer artifacts, and will consequently be unable to function properly.
You should not take this to an illogical extreme—you won't be doing anyone but your opponents any favors if you're taking forgettable artifacts over bombs and premium removal spells—but unless you give a little bit to get the cards that your deck needs, then you're going to end up with a pile of cards that will only do what you want it to do a fraction of the time.
In Magic 2012 Limited, themes aren't quite as clear cut as they are in New Phyrexia / Mirrodin Besieged / Scars of Mirrodin, but there are still some theme-dependent decks that can be quite powerful when you commit to drafting a synergistic deck instead of a pile of good cards.
If you open on a evasion creature on the first couple of turns, then follow it up with a plus-sized Blood Ogre and a big Gorehorn Minotaurs, then you're probably going to win unless your opponent is ready with a Pacifism and an Oblivion Ring (or a similarly impressive response).
In order to take full advantage of the bloodthirst cards in M12, you're going to need to reliably get through for damage early (duh!). If your curve starts at three, then Blood Ogre isn't going to be all that special as you will usually have to play it as a 2/2 first striker—and when you do get a chance to play it as a 3/3, it's going to be at a stage in the game where a 3/3 first striker is easily outshined.
An unenhanced Gorehorn Minotaurs is still a reasonable card—after all, people have been playing with Hill Giants for the better part of two decades—but it's far, far better as a 5/5.
So while many decks are willing to skimp on one- and two-drops (particularly if they have a lot of cheap removal, counters, or the like), if you're drafting an aggressive Black-Red Bloodthirst deck it is absolutely vital that you fill the earliest spots in your curve with cards like Tormented Soul, Goblin Fireslinger, Goblin Arsonist, and Duskhunter Bat.
Given how crucial these cheap evasion creatures are for bloodthirst decks, you need to be prepared to spend early picks on them (or else resign yourself to the fact that you won't be getting that many explosive starts, and you won't get full value out of your bloodthirsters).
Devouring Swarm is a nice card to have in any black deck. However, I'll typically take Tormented Soul over it when I'm trying to put together a Black-Red Bloodthirst deck. While I might end up regretting this pick if I don't get many (or any) strong bloodthirst creatures like Gorehorn Minotaurs or Stormblood Berserkers, if I do get the right complementary pieces, I am going to be extremely happy to have every evasive one- and two-drop that I was able to get my hands on.
While I would prefer to wait until the fourth pick or so to grab them, I've second-picked Goblin Fireslingers, Goblin Arsonists, and Tormented Souls over cards like Giant Spider and Wring Flesh, comfortable in the knowledge that the power I was giving up in the short run would come back to me with interest if I got the right pieces for my deck.
Duskhunter Bat acts as an enabler, and it can also be an excellent threat if you have another cheap enabler at the ready (or if you draw it later, and you already have a way to punch through for damage).
Stormblood Berserker, like Duskhunter Bat, can be a very strong enabler (it's almost impossible to block during the first few turns of the game), but the bonus that it gets when it's enhanced is so strong that it can be quite tempting to wait several turns in order to turn it on.
However, if waiting to pump up your Stormblood Berserker will cost you a lot of time, preventing you from getting full value out of the two Blood Ogres that are sitting in your hand, then you should probably just go ahead and cast it in its unenhanced form and allow the rest of your bloodthirst cards to do the heavy hitting.
It's important to keep in mind that the bloodthirst creatures in Magic 2012 are so strong because of their low costs, not because they offer inherently overpowered effects even at higher price points. (If Stormblood Berserker cost five mana, then it wouldn't be particularly good, but at two mana it's pretty fantastic.)
If you try to make each and every one of your bloodthirst creatures full sized, then you will inevitably give up critical turns before ultimately achieving your Pyrrhic victory: getting to cast some "full sized" creatures when they are no better than the creatures that your opponent is casting at that stage of the game.
When They Aren't Enabling Anything
Then you're going to have to switch to your backup plan.
If you have the right cards to do it, you can abandon your Goblin Fireslingers and build a more controlling Red-Black deck that looks to attrition its opponents out with cards like Gravedigger, and Mind Rot, or simply looks to use its removal spells effectively before ultimately taking over the game with something like Sengir Vampire.
But if you don't have the removal spells, the card advantage spells, and/or the finishers necessary to build a good control deck, then you're going to have to stick to beatdown—even if you don't have the cards that would allow your beatdown deck to fire on all cylinders.
If, towards the end of pack 1 or pack 2, you realize that you don't seem to be on track to put together a dedicated Bloodthirst deck, then you are going to need to start putting together a back up plan.
If you get to the end of the draft, and you don't have any plan for how you're going to grind out wins, then you're going to need to run some particularly
You can pick up a few Lava Axes or Goblin Grenades to burn your opponent out with. Or you might snatch up all the Fiery Hellhounds, Goblin Tunnelers, and Flings that you can get your hands on and try to steal some wins that way.
No matter what your backup plan is, you should make sure you have one—particularly if it doesn't seem like the draft is going quite your way.
If you take the necessary enablers early, you'll be free to draft whatever cards you want later, instead of scrounging for (often suboptimal) roleplayers.
You see a late Wring Flesh? Great! If you have the choice, it's far better to take an extra piece of spot removal (which can also be used as a combat trick) than it is to take a Goblin Piker unless you desperately need another cheap creature.
You can also use your late picks to grab cards for your backup plan. So while it might seem like taking those enablers early boxed you into a corner, you can use the fact that you've already developed a key area of your deck to branch out in ways that can protect you if things don't go exactly according to plan.
The Wrong Enabler
At first glance, Goblin Tunneler seems like an excellent addition to a bloodthirst deck. If you have a Goblin Tunneler, you'll be able to punch through for damage with any of your little creatures, turning on bloodthirst and nibbling away at your opponent's life total.
In reality, it doesn't play out that way. All of the one-drops, and most of the two-drops, that you want to play in a Black-Red Bloodthirst deck already have evasion. Goblin Tunneler is at its best when it's making Fiery Hellhounds unblockable, allowing you to take huge chunks off of your opponent's life total even through a completely deadlocked board.