In M:tG, the basic rule is that, for stability's sake, you want as few cards as possible in your deck, and the fewest different cards in your deck. For example, in traditional tourney rules, you want a 60-card deck (the minimum size), and 4 iterations of each card (the maximum allowed, which means the fewest different cards in your deck.)
I've seen longtime CCG players like ScaredGirl apply that same reasoning to Elements, and I'm going out on a limb and saying that it doesn't apply here. The rules and the card supply are simply too different.
By card supply, I mean that in Elements, especially after the Sundial correction, it's almost impossible to build a deck that wins by any method other than creature attacks. There are few Firebolt killers and fewer Eternity deckout machines nowadays, because the stalling provided by Sundial was pretty much mission-critical to both of those.
That means that, realistically, the only threat you need to deal with to survive is "creature attacks". On the other hand, the only threat your opponent has to deal with is "creature attacks" -- and there are a wide variety of ways to deal with "creature attacks", from Otyugh consumption to shields to Firestorms and Plagues to simply having a faster or more unstoppable creature attack.
Where the winning happens, then, is not in your ability to perform "creature attacks", but in your ability to counter your opponent's method of countering your creature attacks, and prevent him from countering your own counter-creature-attacks. (To this end, Quintessence has been huge, as Quintessence+Otyugh is a very hard-to-stop counter-creature-attack,and decks that can get that combo out first are at a huge advantage.)
The point is that in Elements, it's not enough to have a solid plan of attack -- you need a solid plan of counter-defense to go with it, and because defenses come in such a variety, you need a variety of counter-defenses. Permanent destruction to stop the shields and healing, creature destruction to stop the fast attackers, and untargetability to stop the 'active defenses' like Thunderbolt and Eagle's Eye.
To that effect, I am firmly of the opinion that to survive in the PVP and Lvl5+ milieu, a deck has to have stability in it's offense, but variety in its counter-defense. A 30-card deck that packs 6 Explosion for counter-defense is going to perform less well in general than a 30-card deck that packs 2 Explosion, 2 Thunderbolt, and 2 Quintessence for counter-defense.
Stability is critical on the attack -- if you don't have a strong plan and the ability to execute it, go home. Similarly, if you can't get a strong defense up (even if that defense is "attack faster and harder"), you're doomed to fail. But that same logic no longer applies when dealing with counter-defenses, and that element of Elements is very important when you're dealing with the upper-level game.
