So many intricacies in SC2. Many many threads could be created about those subtleties.
I'm a Zerg player, but I usually random now. Let me warn you that going the path of the Swarm is a lesson in humility.
You're going to get stomped by cheesy all-in bull both Terran and Protoss excel at in the early and midgame, although its gotten better over time - Both Protoss and Terran have received several nerfs since launch; Zerg has received only buffs. The most recent buff, changing spore crawlers root time from 12 to 6 is a big one. Makes Banshee/void ray cheese a heckuva lot easier to deal with.
But other stuff like a quick 1gas 4gate with a good proxy pylon, proxy barracks/factory/starports/gates/stargates/etc, cannon rushes, 3rax-all-in, 2starport banshees, 2gate, scv-all-in, etc will make you want to cut some Blizzard employee's throats out before its all said and done.