I've been thinking about this for a bit, and I think there's a simple way to break down the matchup-based aspects of strategy. What do you guys think of this table?
In other words, we want to win when we're strong and our opponent's race is weak.
So, in a ZvP, Z wants to win in the mid game, while P wants to end in the early game. In a TvP, T wants to win in the mid game, while P wants to win in the late game. In a ZvT, T wants to win early, while Zerg wants to win late.
So, in every matchup other than mirrors, there's a period where you're favored, a period where the other guy is favored, and a period where it's anyone's game.
I think that's why a bunch of ZvTs are so crazy, since so many of them make it to the mid game, where both races are evenly matched. I also think it's why Protoss cheese dominates Zerg--they're winning in the early game, when they're strong and Zerg is weak. Come to think of it, that's also why commentators get severe excited diarrhea during ZvP late games, when Z has a bunch of bases and floods and vipers and brood lords, while P has a death ball.