What I present is my approach to deckbuilding. It is one that has been refined through discussion with a number of great deckbuilders and players, and while not the only way to look at deckbuilding, it contains a number of principles upon which most good players agree.
Keep in mind that the basic assumption of this entire series is that you build decks to win, not to have fun. If you are interested in building fun decks, I would suggest trying out new things with seldom used cards or trying out theme decks: a rat deck, a gorilla deck, etc. Wacky combinations are fun if you could pull them off. However, if your aim in building decks is for tounaments and for winning consistently, then the following are general principles which I have found to be most useful.
One is that a deck should have no more than one primary color. "Primary" in this case means spells of that color would require more than one mana of that color. For example, if I am playing a R/B deck, I would shy away from using Hypnotic Spectres (BB1), Black Knights (BB), Drain Life (B1 + B/pt of damage) AND Shivan Dragons (RR4), Volcanic Geyser (RRX), and so forth. Each of these cards is useful and powerful and are seen in tournament play -- however, all of them require two colored mana to cast. Even with 4 Sulfurous Springs, 4 City of Brass, and 4 Lava Tubes, you may find that you are holding cards waiting for a certain type of land to appear. A well-timed Stone Rain or two may send you spiralling into defeat holding a fistful of powerful cards. A better strategy would be to make black, for example, your primary color and red your support color, and use only the red spells which require one colored mana, such as Fireball instead of Volcanic Geyser.
This conclusion, however, is currently under challenge in the Type II environment. Two developments in Type II -- the printing of Thawing Glaciers and the restriction (and subsequent ban) of the Strip Mine -- mean that it is no longer a matter of great difficulty to get the colored mana necessary given the time. The best example of this can be shown in the incredible strength of what is commonly called the CounterHammer deck, which utilizes Counterspells (UU) and Control Magic (UU2) along with the Hammer of Bogardan (RR1). The Hammer requires RRR2 to bring back into one's hand from the graveyard, while many of the critical blue spells require UU in the casting cost. Without Thawing Glaciers, or with 4 Strip Mines in every deck, the CounterHammer might very well have an extremely difficult time because of the colored mana needs. Instead, it fares extremely well in today's Type II environment.
Nonetheless, it is necessary to realize that even in today's Type II environment, it is still more advantageous to choose spells which require fewer colored mana in its casting cost, and which require less in overall casting cost. It is still better to choose Fireball over Volcanic Geyser unless there is some other reason to use the Geyser (e.g., you need to have instant-speed X-spells). Another caveat is that when Ice Age leaves the Type II card-set, it will take the painlands with it (at least until 5ed, according to rumor). The lack of dual-lands, even allied ones with penalties, reduces the deckbuilder's ability to provide the correct color mana for his spells.
Another conclusion which follows from the principles is that every deck should have an open-ended combination which will supercharge the basic strategy, if you will. For example, one could imagine a speed deck with lots of green weenie creatures. The basic strategy is to overwhelm them with creatures. Adding just one Kaysa (green legend which gives all green creatures +1/+1) is an open-ended combination, as she is useful alone, and each Scryb Sprite is useful in and of itself, but together, you can have 2/2 Scryb Sprites attacking for twice the damage. Another example is Drain Life and Lake of the Dead -- the Drain Life is clearly useful in and of itself as direct damage and as creature-removal, and the Lake of the Dead is useful as a burst-mana producer. Together, they turbocharge the basic Drain Life into a huge swing -- 12 or 13 point Drain Life is not unheard of in competitive play. These are open-ended combos which take a powerful strategy and make it even more effective. The trick is to identify what combinations are indeed open-ended and what combinations are in fact dependent, so as to render one or more of the components less than maximally useful in the deck.
The First Principle, of maximum card utility, has this corollary: neutralize every class of threat. Any serious tournament deck must be able to deal with the four classes of threats: creatures, artifacts, enchantments, and now with the advent of the Kjeldoran Outpost, lands. Simply overwhelming their resources with speed and power is one strategy, and it is a way of dealing with all four of them: offense may be the best defense. In general, however, decks will attempt to neutralize each class of threats, some better than others. But every serious tournament deck must have some way to deal with all four of them.
In addition, in terms of evaluating individual cards for inclusion, the corollary means that Disenchant is superior to Divine Offering and Pillage is superior to Shatter. Any card which is able to deal with multiple classes of threats serves the First Principle more effectively. It is an obvious point, but not everyone understands why it is so obvious.
The Second Principle, of minimizing combo reliance, has this corollary: pose multiple threats. It goes without saying that each card in a tournament deck should be maximally useful by itself (see the First Principle). But in addition, every serious tournament deck must be able to pose multiple threats. The pure blaster deck poses multiple threats in that overwhelming speed is a threat. Countermagic is a defensive threat since it prevents the opponent from playing her game. When combined, the multiple threats may form devastating combinations, but independent of any combination, each threat should be a credible danger.
For example, an Ivory Gargoyle by itself poses an offensive threat. Jokulhaups poses a serious defensive threat as it will wipe out all permanents besides enchantments. Together, you have a combination which deprives both players of all permanents but gives you a 2/2 flyer at the end of the turn. But each card is a self-contained threat. Contrast, for example, a Disrupting Sceptre and the Rack. Together, they can lock up a game almost completely and result in victory in short order, but each is not a credible threat. The Disrupting Sceptre is a credible defensive threat, as it forces the opponent to discard a potentially useful card each turn it is in play, but it can do very little to stop spells being cast, or remove other threats in play. The Rack is a credible offensive threat only when the opponent has very few cards in hand and no way to replenish it (e.g., because of a Disrupting Sceptre) but outside of such a circumstance, the Rack does very little either to disrupt the opponent, to deal with opponent threats, or to damage the opponent very much. A better combination would be to use a Disrupting Sceptre together with counterspells, as the latter can stop what the former cannot and is a credible defensive threat in and of itself.
Defense is a threat. There are a number of decks which focus on achieving and maintaining a lock of some sort. A Disrupting Sceptre/Counterspell lock is one, and a staple of the so-called Weissman deck. Winter Orb/Icy Manipulator is another lock and a staple of decks known as The Prison. Both decks are notoriously short on offense -- a couple of Serra Angels or a single Black Vise and a Feldon's Cane might be all. Yet both are extremely effective because of the multiple defensive threats: counterspells to prevent, extremely efficient removal in Swords to Plowshares and Disenchant, global sweepers in Wrath of God and Armageddon, disruption in Disrupting Sceptre or Winter Orb, etc. etc. Each theat is a credible and effective defense, and combined together, they create an environment in which the meager offense can be brought on when it will be brutally effective.
The Third Principle, of minimizing mana requirements, has this corollary: overload the mana supply. The flipside of minimizing the color requirement is to make sure that your colored mana supply is more than adequate to support the cards drawn. The general rule of thumb cited by the Rulebook and other sources is that a deck should have 1/3 mana -- in a 60 card tournament deck, that translates to 20 sources of mana. The corollary to the Third Principle states that 40-45% is the proper ratio of mana-to-spells, which is broken down to colored mana support in the range of 33-40%, depending on the deck, and 5-10% "alternative mana".
What does this mean, exactly? Colored mana support is the lifeblood of the strategy. The goal is the ensure that no matter what card is drawn, it can be cast without being concerned about being color-locked (i.e., when you don't have the requisite colored mana to cast the spell). In a deck where only two spells cost WW (let's say Wrath of God), one might include upwards of 14-16 sources of white mana, typically using multilands of one type or another (painlands, depletion lands, or City of Brass). If one includes a third or fourth "splash" color where you have only one card in that color, the mana ratio should work out to 3-4 sources of that color mana. In terms of total mana ratio, "alternative mana" refers to things which may not be colored mana but could become colored mana (i.e., Fellwar Stones, Sol Grails, Birds of Paradise, Fyndhorn Elves, Dark Ritual, etc.), could get colored mana (i.e., Land Tax, Thawing Glaciers), or do double-duty as colorless mana (Mishra's Factory, Strip Mine, etc.).
A 60-card deck with 10 basic lands in the primary color, 4 City of Brass or other multi-color lands, and 6 basic lands in the secondary color gives you 14 sources of primary color and 10 sources of secondary color. In addition, 4 Mishra's Factories and 4 Thawing Glaciers add 8 "alternative mana" sources for a total of 28, or about 46% of the deck. This is adequate for most deck strategies. Others, however, may go up to 32 or so by including things like Fellwar Stones, the various Diamonds, or Elves and Birds of Paradise. Some decks favoring a speedy attack over anything else might go down to 18 mana sources or so, including alternative mana sources. The tuning of the mana ratio is the most difficult and demanding, as well as the most masterful and important, deckbuilding skill.
In addition, in today's Type II environment, there are certain individual cards which bear consideration. Some are capable of swinging a game from victory to loss -- these "swing cards" are usually global sweepers such as Wrath of God and Armageddon. Others are capable of winning a game by itself and are extremely difficult to deal with effectively.
Swing cards which are common to the Type II scene are Armageddon, Wrath of God, Nevinyrral's Disk, and Seeds of Innocence. They can be dealt with usually by playing around them -- for example, by holding land if you suspect an Armageddon -- but they should be taken into account if possible, for example, by including non-land sources of mana or non-creature sources of offense.
The "win" cards are different. These must be taken into account when building a deck. The Hammer of Bogardan, Kjeldoran Outpost, Sacred Mesa, Forsaken Wastes, and others sure to be forthcoming in future expansions, must be considered. If the standard strategy cannot cope with any of these "win" cards, then the sideboard should step in to help. The most obvious example is the Hammer. If not playing with Dissipate in the main deck, it might be wise to consider them for the sideboard, if other means (such as Tormod's Crypt or Ebony Charm) are not used. Or, if your basic strategy is to win quickly before the Hammer-recursion becomes a problem, then the card can be ignored, but you should keep in mind that you are essentially conceding the game if the Hammer-recursion starts to happen. Such play-method amounts to, in my opinion, to rolling dice.
Finally, here are some questions to think about in any deckbuilding exercise: