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TL:DR
1) Zerg not being able to scout Terran (hard to scout with slow overlords against decent terrans).
2) Scouting is life-or-death for zerg b/c Terran has such versatility of opennings that all control Zerg BO ("force", accordin to day9's terminoloy)
2b) Zerg are "forced" don't react properly, and they die. like the concept of autowin.
4) It doesn't go the other way from Terran's shoes b/c there's not too much Zerg can do to put Terran in a life-and death situation b/due to wall-ins, and perfect scouting (scans)
* So, given this disparity in the severity of mistakes, shouldn't zerg be able to scout terran batter than terran can scout zerg, and not the other way round?
*side note:
- Terrans not using orbital energy is much more forgiving since they can still mass mule, get the money (they only lose opportunity cost), and still be ok b/c TvZ is defensively strong.
- But zerg's larvae inject, if you miss it, equates to wasted energy that can't be reused without making more hatcheries.
1) Zerg not being able to scout Terran (hard to scout with slow overlords against decent terrans).
2) Scouting is life-or-death for zerg b/c Terran has such versatility of opennings that all control Zerg BO ("force", accordin to day9's terminoloy)
2b) Zerg are "forced" don't react properly, and they die. like the concept of autowin.
4) It doesn't go the other way from Terran's shoes b/c there's not too much Zerg can do to put Terran in a life-and death situation b/due to wall-ins, and perfect scouting (scans)
* So, given this disparity in the severity of mistakes, shouldn't zerg be able to scout terran batter than terran can scout zerg, and not the other way round?
*side note:
- Terrans not using orbital energy is much more forgiving since they can still mass mule, get the money (they only lose opportunity cost), and still be ok b/c TvZ is defensively strong.
- But zerg's larvae inject, if you miss it, equates to wasted energy that can't be reused without making more hatcheries.
Currently, there has been a lot of analysis trying to explain why, in their perceptions, ZvT is too strongly in favour of the Terran. Explanations range from Terran units too cost effective against zerg (e.g. Thors, Hellions, etc), how they hard counter what zerg has while is only soft countered by a zerg composition, map imbalances, and maybe terran adavtages in economy (specifically related to the mules, as articlulated by Dimaga in the post-IEM interview). While I agree with most of these explanations, maybe in particular to the last two, I think that the most broken aspect of ZvT is the scouting, and this is compounded by the fact that Zerg has to react perfectly, which requires good scouting, in order to stay alive against terran.
As a player who at least have a reasonable understanding of this matchup, I've practiced a lot of games against decent Terrans (by decent I meant 600-800 diamond, so i'm not trying to say that I have a lot of credentials here), and, what i found out is that, whenever I ask them to do a specific build against me, I would have no trouble countering it. This is not to say i win the game in the long-run, but that I would be able to survive at least the first push and have a decent chance at winning the game. This made me think for quite a while, and, upon watching many pro-level replays, reached a conslusion: Terran is imbalanced against zerg not because Terran units are just that much better, not (mostly) because of the maps, but because Terran knows exactly what zerg is doing because of a perfect scouting tool (scan), while the zerg has to guess unless if the terran makes the mistake of not denying the slow overlord.
As we understand, Terran has a lot of openings, and, in order to stay alive zerg must react perfectly and uniquely to almost each one. Mass reapers? get roaches. A pair of reapers followed by MMMs? speedlings then bling/muta. Fast thor? bling/ling/muta. Fast Hellions? roaches. Fast Hellion/ MMM push? Roaches + Ling/bling. Terran FE? Muta then expand again yourself. Banshee? fast spire.... List goes on and on, but the point here is that, if zerg reacts correctly, zerg can survive the first crucial attack without much trouble. But, however, what if you react incorrectly? just how severe are the consequences?
If Zerg fail to adapt to what the terran is doing, it is half the time an autoloss, or at best you are so behind that your chances are very slim. Imagine making roaches vs the first few reapers, because you think they hid more reapers, but 4 minutes later to have a huge MMM deathball arriving at your frontdoor... Making fast spire against a MMM push because you found the starport which was actually used for medivacs instead of the hypothesised banshees is also detrimental to your chances.... But, for the terran, on the other hand, not reacting correctly does not result in autoloss, in fact far from it. If your first Thor push gets killed, zerg usually can't end the game immediately because of the wallin. The best Z can do is force some turrets and maybe grab an expansion, while the terran can still grab his expand via a planetary fortress. The game is only even, or slightly in Z's favour, if terran's strategy is foiled by the zerg. Now some may say this mechansim is unfair, but insofar as I am concerned, this isn't a problem if, and only if, zerg have some way of knowing with a degree of certainty the Terran early/ midgame strategy. That is where the problem is.
At the moment, zerg cannot scout a good terran who can kill off the overlord quickly enough that he cannot reach the relevant tech structure. Zerg has to wait till lair and blowing 100 gas in order to know what is coming. against certain builds, e.g. fast Banshees, Thor push, and definitely the mass reaper, this is already too late. This means that zerg has to, in effect, guess what is going to be thrown at him, with the correct guess meaning that the game can go back to roughly 50/50, and the wrong guess meaning an instant loss. With the dozens of openings T has available, and how each opening counters many of the counters of another opening, the majority of TvZs end with the terran victorious in the early stages of the game. the fact that zerg has to guess is compounded by a terran advantage: the scan.
The scan is the most viable early-game scout, and definitely the best scout out of both races in a TvZ. What's the issue? well, if zerg has to guess the right counter to a T opening in order to stay alive, the fact that Terran can know with a fair amount of certainty what this guess is means that T has the indisputable advantage. Terran can scan, know what Zerg is trying to counter, decide to go for a build that counters exactly what zerg is doing. This is "helped" by the fact that Terran structures train so many different types of units, while for a zerg, it is obvious when he puts down a roach warren instead of a baneling's nest that he intends to make roaches, not banelings, which makes identifying Zerg's strategy using scan very easy, and idenifying the terran strategy, not even considering that Z cannot scout T effectively, hard for the zerg. The fact that zerg cannot scout terran and therefore has to guess, and that terran knows exactly what zerg wants to do, means that terran is far far more likely to choose the optimal strategy. This brings us back to my first point: that a correct strategy for a terran, and therefore a wrong guess for the zerg at this stage of the game, is so so unforgiving as far as Z is concerned, since it usually either autoloss the zerg or sets him back so much that defeat is very likely.
For the terran, however, as previously discussed, getting a strategy wrong isn't the end of the world. It just means that the game is more or less even, and that they have to play a longer macro game to win. Perhaps some terran strategies (e.g. the fast banshee followed by the cloak upgrade) is less forgiving than others, but generally the terran builds are not all-in in the sense that there is very reasonable hope of recovery. So, given this disparity in the severity of mistakes, shouldn't zerg be able to scout terran better than terran can scout zerg, and not the other way round? In that way, a zerg player not focusing on the game will still be punished heavily for his poor decision-making, but a zerg playing well would be able to at least scout the terran opening and survive the midgame.
I will now discuss the other causes of perceived imbalance according to others. Maps is indeed a big issue. Perhaps the most famous (infamous) example is the Idra vs Silver game on Lost Temple where Silver abused the natural cliff to the fullest using fast siege tank drops. It must be noted, though, that if Idra knew what was coming, he would have got fast spire or drop upgrades. In maps with smaller open areas that favour early mech pushes, zerg would get faster mutas followed by a faster hive if they know that T is going for this strategy. The point is that maps do complicate things by adding to the amount of options T has, but these additional Terran options are surmountable, if and only if Z knows what is coming.
Terran units are perceived by zergs to be too strong. E.g. there are complaints about the amount of damage reapers do even to spine crawlers and hatcheries, about how siege tanks don't waste dps, and about thor dps against both ground and air, about the planetory fortress and Terran mech in general. I think the problem isn't the units themselves. Sure, many have scary stats or are simply so hard to kill when scvs are constantly repairing them (read thor and PF), but the main issue is still what is going to be thrown at you. If you know what is coming, you can at least have a chance of defeating it. I'm not saying that Terran units are fine, because I do harbor some bad remarks about the thor and the new missile turrets, not to mention the double-up medic/dropship unit, but that the most fundamental issue is the scouting.
Mules are another phenonmenon altogether. The Dimaga vs Lucifron series comes to mind. In the game on Lost Temple, Lucifron came back against Dimaga economically after losing all his scvs by cashing in his orbital command energy for mules (though Dimaga's actions after his scv killing spree is questionable). Mules allows Terrans to forget about their Orbital's energy but yet still be able to use them for macro later on. It is too forgivable. I've seen so many replays where the T has economic parity with me while we are 5 workers and one base apart. The lost energy on a queen is just lost indeed.
The main issue, however, is zerg scouting. Zerg lives and dies on whether or not it reacts properly to Terran openings/ midgame startegies, and at the moment we have to guess while the T knows very well what that guess is due to their scans. A half-decent Terran will never let your slow overlord garner any useful information pass the first few minutes of the game. You will be lucky to see the tech lab, let alone what the lab is attached to. Terran does not live or die on the scout, and a wrong strategy by the terran merely results in a more even game most of the time. T scouting is way too effective. This is compounded by the fact that it is easy to identify zerg strategies by the buldings you see in a scan (if you see a Bling Nest you know it won't make Roaches), whereas terran strategies are harder to identify. (rax with a tech lab makes both marauders and reapers, not to mention you can exchange tech labs around with other buildings). If Zerg has better scouts, it could also lead to more interesting, longer macro games, since Terran will be far more like to Fast Expand since the usual one-base play will be less rewarding. So: In order to balance ZvT, the first thing that must be done is to give zerg better scouting tools early on, since one wrong decision is fatal for zerg.
edit: I'm not too sure what you meant waffling1, but I assume you mean I should paste a simplified version at the top of the page? If so, thanks for that, very good idea. *changed the TL: DR a bit over the last point, since it is what I actually wanted to say.