Right now there is a movement that is moving accross Starcraft 2 and Battle.net. It promotes the idea that the Zerg are the weakest race in the game, and don't even stand a chance against Terran or Protoss opponents. They give many reasons: how difficult it is to control Zerg's economy, their scouting issues when players block off, how there aren't many professional Zerg players (a fallacious argument), and more. These arguments may or may not be true, it's too early into the metagame to tell. However, the Zerg ARE a force to be reckoned with,
When it comes down to StarCraft 2 strategies, the Zerg by far have the most diverse options. Why is this? Mainly because how their units are made. Unlike the Terran and Protoss who make their units from different buildings, all Zerg units are made at the same one: the Zerg Hatchery. This can make it very hard to balance your economy with your ability to fight your opponents because both your workers and fighters must share Larva (make sure to get a Queen to make more Larva, and make sure to expand early). Because of this, the Zerg are generally a reactive race, and their strategy options are nearly limitless.
Terran strategies and Protoss strategies are much more limited because you must make the first move while the Zerg watches and waits. And if you don't make a move quickly, the Zerg's economy will evolve rapidly and grow out of control. While you build up your force, the Zerg watches and develops a strategy to counter yours. It can be very difficult to get a significant advantage against a Zerg opponent.
I think the main reason people have trouble playing Zerg is because they are playing aggressively rather than reactively. If they tried to play the waiting game and adjusting to their opponent, they would likely have a much higher success rate.
