http://www.starcitygames.com/php/news/expandsub.php?Article=1683

Part 1: Threats
While combo decks are weak in Type 1 due to the restriction of Tutor cards and the large number of control decks, most decks in Type 1 have a very pronounced fundamental turn that gives most decks a "combo-esque" feeling. By turn 4, Suicide Black has overwhelmed you, Sligh has burned you out, or Keeper has completely shut down your creatures with The Abyss. This facilitates the need for fast, disruptive, or durable threats. Fast threats are those such as Jackal Pup, Phyrexian Negator (with Dark Ritual), or Juggernaut (with Mishra’s Workshop). Each of these cards is capable of doing significant damage before an appropriate answer is found. Disruptive threats are those such as Hypnotic Specter, Gorilla Shaman, and Meddling Mage; these disruptive cards make it more difficult for the opposing player to remove them or to win (whether literally or figuratively). The last set of threats are durable threats: These are the cards like Morphling, Blastoderm, and Cursed Scroll that allow a player to win around an opponent’s defenses.