Direct damage decks tend to be very fast, and very furious, trying to take out their opponent as quickly as possible. A rather "go-for-the-jugular" approach to deck design. The main cards to decks of this nature are red, including Lightning Bolt, Chain Lightning, Incinerate, Fireball, Disintegrate, and many other spells. Direct damage from other colors include blue's Psionic Blast and green's Storm Seeker.
Due to its destructive nature, red seems to be better protected against than the other colors. However, the advantage to these decks is their quick-kill capability. Unfortunately, with all this quick destruction, one card per turn is seldom enough for these decks. Often, you will want additional cards to be able to do more damage to your opponent more quickly. Some of your options include Howling Mine, Jayemdae Tome and Elkin Bottle. The Howling Mine allows you to draw one extra card per turn for free. However, it also conveys this benefit to your opponent. The Jayemdae Tome is much slower, costing twice as much as the Howling Mine to put into play, and costing 4 to use. However, later in the game, the cost of 4 is not bad when you consider what you will be casting will either be a lightning-bolt type spell (1 or 2 mana) or a fireball (which you could theoretically hold until the following round for maximum damage). The Elkin Bottle is good in that it costs one less than the Jayemdae to cast and use, and most of the spells in your red direct damage deck cost one or two mana, thereby reducing the risk that the card you draw will be removed from the game. However, if you decide to incorporate some of reds more expensive spells and creatures, you may want to take one of the other options.
The downfall to these decks, of course, is that red has so many cards against it, that are common, and most anti-red cards are inexpensive to cast. Circle of Protection: Red is the most infamous and difficult to get around once it is in play. Conversion and Justice are others. However, not all the anti-red cards are in white. Blue has its fair share.
One of the variations of the direct damage deck a red and green deck designed to kill your opponent quickly with the aid of green's mana producers, which has become somewhat less popular with the banning of Channel. Some of the best cards for this kind of deck include Orcish Lumberjacks, Birds of Paradise and Tinder Walls. Fire Sprites may work well too, converting your green mana into red when needed.
If you play a direct damage deck, make some provision to take out enchantments that allow your opponent to stay alive. The green/red deck has access to Tranquility. Other cards include Anarchy (specifically against white) and Disenchant.