I've seen several people comment about sending "harassment" units to attack the opponent's starting Mana Crystals, but unless I'm totally mistaken this doesn't seem like a great idea.
A Mana Crystal costs 5 mana, gives 1 mana/turn, and has 6 HP / 2 Armor.
Therefore, if you cast Mana Crystal on Round 1, you'll break even at Round 6 and get a Mana surplus by Round 7.
The fastest way to rush the Mana Crystal is with a Bitterwood Fox (5 Mana, Fast, 3 dice atk) or Thunderift Falcon (6 Mana, Fast, 3 dice atk). A 3-dice attack deals an average of 1.81 damage against 2 Armor. It takes an average of 3.3 hits to kill the Mana Crystal.
If you cast a Fox/Falcon on Round 1, it can move 3 spaces on Round 2, and start hitting the Mana Crystal on Round 3. This means that the Mana Crystal is expected to die around Round 6 or 7.
Therefore, even without any attempt by the enemy player to kill your Fox/Falcon, the Mana Crystal won't die until it's already returned its full Mana cost.
On the other hand, if the enemy player kills your harassing Fox, you're down a tremendous amount of mana. Not only do you lose the 5 Mana from the fox, but your opponent is gaining a +1 mana advantage every single round.
Am I totally crazy, or is the "zergling rush" really not that great at crippling an economy?