EPTBalance, my few remaining friends, is easy. Take a coin, stand it on its edge - bingo: balance. And right now, that's exactly how SC2 is balanced. What's lacking in cetain matchups is stability, which is much more important. Stability means players have options. They can play it safe, or they can take risks to gain an advantage. They can bring their skills to bear to cut corners lesser players could not.
So what does it mean to say that a matchup is stable? Well, a game of Starcraft is essentially an arms race, and an arms race's stability at any given point primarily depends upon:
1. Defender's advantage.
2. The specificity of tech / the degree to which quantity can substitute for the 'right' tech.
Before we go any further, it's worth asking: is it possible to have too much stability? Absolutely. After all, we do want someone to actually win. Ideally, stability should ramp up quickly (so as to permit the opportunity for cheese, but not allow it to dominate) and then decay as the game progresses, both in terms of defender's advantage and the ability for quantity to stand in for appropriate tech. We also want that stability curve to be as symmetrical as possible on both sides of the matchup. If one race has a much stronger defender's advantage, that should be offset by stricter requirements for producing the right tech in the right quantities.
When we apply this thinking to SC:BW, we can immediately see how well it works. Zerglings are strong in defence against marines and zealots, but weak in attack where a good surround can easily be denied by building placement. As the arms race progresses, marines are able to hold off Mutalisks while turrets are made, Vultures are able to hold ground while tanks are being massed, lurkers delay pushes - you know, all that good stuff. At the ends of the tech trees are the units that finally nullify defender's advantage: defilers, guardians, battlecruisers, arbiters, carriers. And the races with the strongest defender's advantage (Terran, Protoss) have to be very careful to keep up with tech and macro so as not to allow Zerg an unassailable economic lead.
When we look at certain matchups in SC2, on the other hand, we can see how they violate our ideal of a symmetrical stability curve. In TvZ, Terran enjoys a distinctly greater defender's advantage prior to Hive tech. But far from this being offset by a need to tech correctly in response to Zerg play, it is Zerg, lacking any meaningful defender's advantage, who is obliged to get exactly the right tech in exactly the right quantities so as to avoid losing immediately or being economically crippled, either self-inflicted or through harassment.
This state of affairs comes about thanks to Terran's well-stocked larder of aggressive options, ferociously rapid and economical tech-tree (everything bar ghosts and battlecruisers can be unlocked within 185 seconds of the first barracks going down for around 650m/350g), and the fact that for Zerg, core functionality like shooting upwards is only attainable by either delaying tech (massing queens), committing to a unit which in all other respects performs poorly in the matchup (hydra), or massively out-macroing the Terran and hoping a timing push doesn't come (Mutalisks).
Fixing ZvT means equalising the stability curve. Terran should need to care more what his Zerg opponent is doing and prepare an appropriate response - this is fair because his defender's advantage is so much stronger in the early game. Zerg, meanwhile, should be given a core unit or unit combo which is reasonably good against a broad spread of Terran strategies, to compensate for their 'open door' policy.
What would I suggest? Well, I'd seriously consider refitting Hydras as T1.5 tech: cheaper (say 75/25 and 1 food), lower DPS, and simply biological rather than 'light' (so as to cope with reapers and hellions). I'd also consider giving them a modest plus-to-armour, to allow them to deal with small numbers of marauders or fast tanks. Meanwhile I'd move Roaches to T2 and re-evaluate their base armour, DPS, regeneration and whether tunnelling claws should be an upgrade.
Why do I think that would help? Firstly, giving Zerg a solid core unit combo (ling/hydra) that excels on creep (ie in defence) boosts their defender's advantage and mitigates Terran's outrageously flexible tech options. Secondly, it gives Zerg more aggressive options at T1.5 (Terran would need to be vigilant against the possibility of hydra aggression taking down their wall-ins) and T2 (Terran would have to be wary of burrowed roach aggression, whilst not overcommitting to marauders/tanks in case Mutalisks are on the way.
Some people want to improve Zerg scouting rather than altering units. I used to think like that too, but then I realised it wouldn't actually fix the problem. The terran player would still get to do exactly whatever the heck he likes, and the Zerg would still have to execute precisely the right response in order to survive. Not to mention that tech labs and reactors make it trivial for Terran to kill an overlord scout and then tech-switch on a dime. No: Zerg needs what the other races have: a solid, dependable core unit combo that can see them through the early game and enable a variety of transitions.
Thanks for reading.
EDIT: In response to a really good point about using lings/roaches/queens for defence:
Roaches, speedlings and queens can indeed defend pretty well. However:
1. Queens cannot be mass-produced reactively in the way other Zerg units can (and need to be).
2. Queen production delays teching.
3. Queens cost a lot of minerals at a time when you're trying to afford an expansion, drones and overlords and are in fact probably taking workers off gas to do so.
4. Queens consume two supply and present no credible offensive threat.
5. Speedlings present very little offensive threat in any quantity, early game.
6. Roaches present very little offensive threat in early-game numbers since they cannot attack a wall without exposing themselves to enemy fire.
For these reasons, speedling/roach/queen does not constitute a stable core army for Zerg.
Hydras at T1.5, on the other hand, could be quickly massed, giving Zerg more time to see Banshees coming and keeping the hatchery free to tech to lair for detection. They would present a credible offensive threat in conjunction with speedlings/banelings, since they can take down a wall-in. They would constitute superior reaper defence, adequate hellion defence, and, with speedling backup, be better at holding against the natural marauder transition from reaper.
In short, with hydraling you'd have yourself a solid (if sub-optimal) defensive option if Terran managed to negate your scouting, and he'd have to keep a close eye on your numbers to be sure you weren't planning on kicking his face in - more scans, fewer MULEs (unless he wants to take risks to gain an advantage, of course). You'd also be able to take measured risks, cutting corners or faking aggression to get faster tech or a better economy, rather than having to take the massive risk of leaving huge defensive holes like you do now.
