Below is a link to a replay of a game I recently just played ZvZ - historically, this has been a really weak match up for me for a number of reasons and I went into the game really nervous. The only comfort I held as the game loaded was that it was an initial placement game.
Looking back, although I obtained a victory, I attribute it not to my skill but the fact that my opponent made far more mistakes than I did. I was frequently supply capped, I didn't keep an eye on harvester saturation, my wallet seemed constantly full of thousands of resources (which did come in handy when I massed mutas, but that wasn't my goal... should have used it on tech), I could have expanded a lot more and a lot faster, and above all I needed to scout more.
My opponent, on the other hand: only expanded once, his muta harass was weak, during my initial z'ling and b'ling attacks on his expansion his primary base harvesters completely stopped harvesting for minutes, he lost a lot of units to b'ling attacks, and his resources were never saturated at any point. I also feel that had he been more keen to maintain more harvesters, he would have easily had at least two more bases in the south-east.
I don't really think I deserved a victory in this game... I felt that there were at least two definitive points during which I really felt that he should have won: 1) when he attacked my gold expansion with his roaches - he had unit superiority on the grid and had he maintained a fatter economy he would have had far more units there; 2) when I allowed him to regroup on multiple occasions in his bases, I allowed him a stronger counter to my standing army.
What I'd really like advice on is unit production - I often find a weakness for me is spamming a unit produce button when I am need of a certain unit (ie... spam for more drones, spam for more overlords, or frantically spam for more roaches or hydras, etc). How do I maintain a proper balance between workers and military units as the game progresses? Also, which tech is (besides z'ing speed) is absolutely vital to most Zerg players?
Thanks for the input all!