Meh, I though I'd post in here, even if it may be off-topic by now.
Defining the meta-game
How I define the meta-game would be what "pros" (the omniscient deck-builder that knows all about cards, decks, etc.) consider as "viable" decks in the environment (unrestricted unupgraded PvP aka PvP1, War rules, Rules of the tournament last week, etc). In this way, the meta-game is, as mentioned in the quote by OldTrees, a collection of competitive decks. In a way, each deck in the meta-game must beat able to beat another deck in the meta-game. In this way, how "viable" a deck is comes from how many other "viable" decks it can beat. So, if Deck A wants to "viable", it has to be able to consistently beat Deck B, which was already proven to be "viable." How was Deck B proven to be "viable"? Because it can beat Deck C, and so on. Soon, you have this large tree, with everything starting at the root. So, what's at the root?
The first thing to consider: Rushes
I think the first deck that is considered should be a rush. But not just rushes in general, the first thing to consider is the fastest deck that can possibly be made. Why? Because you cannot build a rush (deck with no control) slower than the fastest rush, or you will lose. Those decks are not "viable", so the only decks left are those with control, ranging from a smidgen to a ton. But what is the control used for, if not to defeat a rush? As you can see, everything starts at the fastest rush.
The rush
So, the rush is usually a collection of 30 gears designed solely for being the engine that plows ahead to victory. As you know, each deck must consist of quanta sources (immolations, novas) and damage sources, whether direct or indirect. The rush is always composed of direct damage sources, solely because those are the best damage sources for speed. That means high attack, low cost creatures, HP being something to ignore. While the rush may be composed of 16 quanta sources (e.g. 4 novas, 6 immos, 6 photons) and 14 damage sources (all the non-photon creatures and an arsenic), any deck that hopes to beat the rush must add a foreign element: control sources.
Countering the rush
However, these decks are also limited to 30 cards, since it is impossible to keep the rush from dealing a total of 100 damage within 23 turns. (Or is it? Introducing: deck-out decks! But that's later.) So, while this deck may have 14 quanta sources (pillars) and 16 damage sources (direct, in mummies and dragons, and indirect, in poisons), it won't be able to defeat the rush unless it has control. So take out a few of the damage sources and add enough control sources to make up for the difference in speed, while keeping the quanta balanced. This may mean increasing the quanta sources (by two pillars), decreasing the damage sources (by 6 poisons), and increasing the direct damage (since the rush only uses direct damage) control sources (by 4 skull shields). What's this, the deck became slower (only 24 out of 30 gears are designed solely for being the engine that plows ahead to victory)? Although it became slower it is more "viable." As you can see, speed really only matters when determining what is the rush and what needs more control.
The control has you
A deck that wins by slowing down the enemy's direct damage while outracing them using their own direct damage can only be beaten by devoting more of your deck to control direct damage. This is one of the main branches of the tree, with each new deck trying to one-up the other. However, the 30-card decks that focus solely on control will later be shown to be less viable, because they're still using direct damage sources to win, which means that they have less direct damage sources that need to be countered.
After I wake up, I'll explain how...
As more and more decks focus more and more and control sources, the average speed of decks becomes slower and slower. Soon, deck-outs become easier and easier, they become more and more "viable."
Also, indirect damage sources, because you're basically turning most of the counter-rush's deck into dead cards.
Also, denial, because slow start-ups mean that you may be able to start-up your denial before you die.