I preffere Protoss, although I play zerg and terran often enough to keep in touch with what they can do. Special ability wise, Zerg actualy has few usefull ones... Broodling is good, as is parasite, and burrowing is a good general ability, but few beyond that I find practical... the defiler's abilities are good, but not ones I use as often as say, a arbiter's statis field. With Zerg, I feel their strength is the brute force of numbers, and the method Zerg produce units that lends to hammering out swarms fairly fast.
300 per hatchery, plus one-time expensies for auxilery buildins... as opposed to say, Protoss who need a 150m/150g stargate for just 4 units, a 150g gateway for the ground units, and another 200/200 base for robots... that's cost PER unit manufacturing building. Zerg can place multiple hives for fairly cheap in comparison, especialy when you're talking 15 hatcheries vs 15 stargates.
Terrans are my weakest race, unfortunately. Beyond doing alright with goliaths and cruisers, with a respectable turret/bunker/tank base defence... I'm not so great. I can wage effective ghost nuke warefare provided I'm not the main focus of attack myself (as in on a 2v2 or 3v3 or something).
I'm simpy most profficient with 'toss

Scout/corsairs rule.
Useless imp? USELESS IMP? I'll give you useless imp. Here, lemme useless imp you upside the head with this BFG5000X! BANG! Useless imp that! Bwua ha ha ha! ph33r