Welcome to the StarCraft II Beta, build version 0.2.0.13891.
I played 10 games as Zerg. I'll talk more about the specifics a bit later, but here are my impressions of the game from a Zerg perspective.
The overall hectic feel that SC1 had is retained in this game. I can't guarantee that everything will feel the same for someone with 300+APM and the multitasking skills to match. But for me, there was always something better that I could do.
The Zerg themselves don't feel finished. At all. There are a number of just flat-out odd things.
Roaches at Tier 1, but without their trademark regen. Indeed, they don't get their regen until Hive tech. What they are at Tier 1 is a Zerg Zealot: 16 damage and 145 Hp, for 75/25/1 cost. I'm not sure what burrowed movement is really supposed to do to these guys, but it isn't really helping. To make them useful at Tier 2, you have to research 2 upgrades, as well as do a lot of burrow micro. That just isn't worthwhile.
Also, Roach regen only works (unless it's the Tier 3 upgrade) when the Roach is burrowed. But this upgrade doesn't give Roaches burrow; that has to be researched separately, thus driving up the cost of Tier 2 Roaches.
Overseers also are very odd. They don't give extra control when made. All they are is detectors that can create Changelings. The Zerg only have 3 spellcasters to begin with: Queens who are stuck in your base casting Spawn Larva, Infestors, and Overseers. And now Overseers are practically useless.
Corruptors have an ability that shuts down any building for 30 seconds. This feels very... strange. It's an AtA unit, but unlike the Phoenix, it's spell has nothing to do with its AtA powers. The Phoenix's Graviton Beam allows other Phoenixes to shoot at ground targets. The Corruptor's corruption ability doesn't. It's simply an alternate reason to get Corruptors. I never got the chance to abuse this ability, but I will in the future.
Ultralisks were nerfed hard. They're basically 600 Hp versions of their SC1 incarnation. No linear AoE attack, no double attack, and no research upgrade for either. Their research is exactly their SC1 research: +2 armor and extra speed. On the plus side they can still burrow and they still have their 60-point anti-building attack.
Having two gas Geysers in your base (that provide half the total gas of one in SC1, so you don't get more gas) is actually quite a boon, particularly for the Zerg. Roaches and Banelings are Tier 1 units. Being able to throw down an Extractor at 25 less minerals than SC1, and only put 2 Drones on it to give you just enough gas to power out a few supporting Banelings or Roaches to compliment your Zergling army. It's not really much of a difference, but you can control your gas income a bit better. The double-geyser also makes gas more valuable: you have to pay almost twice as much to get the same income rate as SC1, so teching has more of a cost to it.
On to the games.
ZvZ: I can't say what this match will ultimately be like, as my only ZvZ match was against someone even less skilled than me. Mutalisks were annoying due to the only GtA being Hydralisks, who are 100/50 and quite strong. I suspect that any ZvZ that survives past the initial Roach spam will likely become a war for air supremacy. Tier 3 tech is utterly useless against someone massing Tier 2: I eventually won with mass Hydra, slaughtering 10+ Ultralisks.
ZvT: I have a pretty solid opening build for this game. I feel it's pretty safe, though I won't know until I play more than 3 games of it.
It's 10-Overlord-Pool. While the pool is making, Drone to 13 and save Larva. At the moment the Pool spawns, you should have ~300 minerals. Enough for instantly 3 Zerglings and a Queen, taking up up to the 18 unit cap.
With Terran Reapers and other tricks, plus the fact that they easily deny you a scout, I find this is about as econ-heavy as you can afford in ZvT. Zerg fast expansions are too easy for the Terran to punish without having some Hydra or Muta around. So you'll probably want to scout the Terran to see if he's expanding. If he did, mass Zerglings/Banelings and attack. If not, expand yourself.
Note: I've since heard that the extractor trick works in SC2. This might be worked into this build.
ZvP: If you're playing any game against Protoss, you're going to see one build 80% of the time: 3-gate Zealot. I played 6 games vs. Protoss, and lost 5 of them (winning only to someone who really had no idea what they were doing). Of those 6 games, 4 players went 3-gate Zealot.
The problem with 3-Gate is Chrono Boost. Oddly enough, it isn't the effect on Probe production that is the problem. The problem is if they save up 100 energy (4-casts worth), throw down 3 gates, and dump all of that energy into Zealot production. That's pretty much death. It's too many Zealots too fast.
I'd heard about this build before the Beta finished downloading. And despite going 0-4 to it, I don't think it's invincible. You just have to know about it. And SC1 Zerg are so used to relying on Overlord scouting and being the aggressor in ZvP that it's mentally hard to take a defensive posture. That means one thing: Drone Scouting. You must know that this is coming before your pool finishes.
Since 3-gate Zealot is a rush build, it auto-kills any fast expand. So you need to know if they're going for it before you expand. Also, Tier 2 is not going to save you from this; you will die while whatever Tier 2 building you're going for is still being produced. If the Zerg are going to be able to deal with 3-gate Zealot, it will have to do so at Tier 1.
Being hyper-defensive probably works. If you throw down 5 Sunkens, you'll probably survive. But it's not going to let you win, because 3-gate Zealot isn't very econ-damaging. You'll be stuck behind your Sunken line in your main, so you'll have to go for Lair tech on one base. This isn't going to work, since they have contained you. They can expand and build a devastating economy.
So you need to do the containing. That means you need units. Since it's Tier 1, it has to be Zergings plus either Banelings or Roaches. I tried Banelings, and they might have worked if my enemy hadn't attacked while my Ling/Bane army was about to attack them. I was out of position and killed. But I don't think Banelings do enough damage per their cost to Zealots (despite Zealots being Light). Also, the Baneling Nest itself costs gas.
The Roach Warren does not. Roaches do of course, but this buys you valuable time. So if you see 3-gate Zealot, instead of instantly getting a Queen with the extra 150 when your Pool pops, get a Roach Warren. You're really going to need the Queen's Spawn Larva, so don't forget to get her too. You'll also need gas. Whether you get an extractor before or after the pool, you'll need one quickly.
Your goal with a Zergling/Roach build is to contain your enemy and expand. You don't want to overextend yourself and get your army cut down, but you want to make your expansion safe. You'll probably need at least one Sunken to help with the defense against the initial attack. This buys you time to Drone up to getting a good gas and mineral flow. Once you have that, pump Lings and Roaches, then go for the contain. Try to periodically enter his base and kill Zealots before they build up into too big of a ball.
I can't guarantee that this will work, of course. But it has a chance.