In my not so humble opinion, the best way to build Commander decks is with a strong set of themes. To make sure that a deck gets the most out of the cards it plays, it should learn to turn dead cards into new resources and card advantage. This is especially true of creatures.
When players start to build decks with strong themes, many naturally gravitate to building around a specific creature type or a creature-heavy build. Creatures have built in card advantage. Once a creature like Acidic Slime has destroyed a permanent it can still be used to swing at players and cause damage. The popularity of this strategy diminishes the advantages it provides. If you want to get more value than your opponents, you have to go one step further than just running creatures with immediate effects.
To put it another way, the less supported tribal decks (not wizards, elves, or goblins) might require you to commit the necessary evil of playing creatures without abilities in order to stay on theme. When backed into the corner of playing vanilla creatures, players should emphasize ways of using those creatures as additional resources.
Coming and Going
As a general rule, creatures enter the battlefield and eventually leave the battlefield. Creature-heavy decks should include ways to get ahead when creatures follow this natural progression. Cards such as Soul of the Harvest, Harvester of Souls, Fecundity, and Skullclamp leverage this natural cycle into part of a draw engine. Cards such as Aura Shards, Martyr’s Bond, and Grave Pact give you the ability to use this cycle to turn creatures into removal. Cards like Birthing Pod and Wild Pair even allow you to turn the process of casting or simply having creatures into tutors. Tribal decks can take advantage of cards like Mana Echoes to gain mana from casting creatures.
These types of cards won’t make a deck great on their own, but they make creatures worth more. Value loops give decks access to greater stability than the above examples and push decks ahead of the pack. This may be something like Fecundity plus Pentavus, which gives a player the ability to pay two mana to draw a card at any time. Keeping these loops going usually requires a strong flicker or recursion package. If you are sacrificing creatures to Birthing Pod to go search for more creatures, being able to instantly bring them back turns that interaction from good to great.
On Board Value
Players should also strive to take advantage of creatures just being in play. The combat phase is the most common way to take advantage of creatures being in play. Card like Edric, Spymaster of Trest and Coastal Piracy enable a player to draw massive amount of cards from having creatures enter the red zone and connect, just as various equipment like the Mirrodin swords give generate all sorts of value.
One of the less common, but incredibly powerful, ways to use creatures is by playing cards that allow you to tap those creatures for resources. Cards like Grand Architect, Earthcraft, and Azami, Lady of Scrolls generate an insane amount of advantage via tapping creatures if used properly. Decks can even use creatures as control elements with Glare of Subdual or Opposition, for example. Those sort of effects usually are most advantageous if a deck contains lockdown cards such as Static Orb and Meekstone or if a deck takes advantage of “tapped” specific removal such as Sunblast Angel or the assassins.
Creatures usually have to hit the battlefield to generate resources. But there are many cards that allow you to discard creatures to get some value from them when you don’t need to cast them. Cards like Tortured Existence and Survival of The Fittest are perfect examples of tools to maximize the value of otherwise-dead creatures in hand. Some creatures are even better off discarded straight to the yard, like Anger and Genesis. Others just generate value because they are easy to turn into free resources, like creatures that return themselves to their owner’s hand from the graveyard, such as Blood Speaker and Squee, Goblin Nabob.
There is a whole series of cards that give you the option to discard or exile cards instead of paying their mana cost. Holding an otherwise dead creature in your hand so you’ll have fuel to pitch turns those creatures back into valuable resources that enable you to interact with the game even if you are tapped out.
The most important thing to remember if you are discarding cards for value is that you need a way to get them back. If you are pitching cards to Mind Over Matter to untap a mana source, you want to be able to return all those cards to your hand or play if at all possible. In this example, if you use the mana to cast say, Rise of the Dark Realms, that would be an excellent way to turn your creatures into other resources.
The moral of the story; if your deck revolves around creatures, make them more than just creatures. You shouldn’t evaluate cards in a deck as individual cards, “in a vacuum.” If you want a deck to run, it needs an engine and fuel to keep it going. In a word, those creatures need to be part of synergy. In Commander, you never know when the game will go on for an hour more. You don’t want to be stuck in “top-deck mode” because you didn’t turn your vanilla birds into more valuable resources.