For example, I'm sure many past chatters mentioned how exploitative Nydus play could be, yet for (many!) years pros didn't use them to great effect. Eventually they caught on and for a while Nyduses were responsible for some pretty broken matchups, predictably resulting in it being severely nerfed, thank [Allah | Crom | Cthulhu | God | Morgan Freeman].
Pros aren't always right nor all that creative, they sometimes do stupid things, and being lesser rank than them, or anyone else, shouldn't necessarily default-invalidate criticisms or recommendations from relatively lower-level players.
The highest rank I've achieved is M2, I'm a low-APM Terran. This post is long yet not exhaustive, and many of the points could've probably been articulated better. I look forward to all thoughts / feedback, even if it isn't constructive. :-)
+ still don't utilize Nyduses all that well, especially late game—I'd love to see them used for taking ninja expos, camping production, etc. It's such a relatively low cost-vs-reward ratio.
+ rarely research burrow, even if for only the sake of being able to save economy, especially in ZvZs.
+ early spines / spores. I'm not sure how "early" they should be, but man, there are a lot of games where Zs take so much avoidable damage. Sacrifice a few drones on early static defense—it won't wreck your economy, but a few sneaky Hellions (still?) often do.
+ more bane drops. It literally took until Serral to see them really utilized. Pretty pathetic.
+ lose some SCVs to run-by / banes, and don't resume SCV production. Instead they oftentimes go all-in. In certain situations, I suppose they have to. In others, especially late-game, they probably don't have to and actually increase the likelihood of losing by thinking they must go all-in, then do so into a defensive Z or P. We know how that usually works out.
+ tank-push against Ps and clump their tanks up. Jesus Christ, spread out your tanks! They should never take Colossus splash, nor should multiple tanks be damaged per friendly-tank shot. (For example, X clumped tanks, Zealots / Stalkers on them, and a one-off tank firing on Zealots / Stalkers, consequently damaging all X clumped tanks.
+ vs Z, widow mine play should always include at least one siege tank to mess with the AI of lings / banes. Additional splash of course helps. The only player I saw do this was Last, who if he had stuck with SC2, probably would've been a world champ. (He's the only player I ever saw completely clown TY, had some of the best vs Z-micro I've ever seen, as demonstrated vs Life, where he had no stim or shields vs speed banes—Day9 was completely amazed, and to date, is the most amazing SC2 micro I've ever seen)
+ not enough factories. Why 3 mines at a time in late game TvZ? Is 3-factory, 4 mine + 1 tank or Thor at a time sustainable? More mech needs integrated with bio if bio is going to work. Alone, it seems the best Zergs have figured out how to utterly crush pure bio.
+ Turrets. Guys, come on. If they go muta, put up a lot of turrets. So many games seem to be lost simply to inability to deal with mutas, and most the time mutas aren't dealt with well early on simply from lack of turrets. Same for warp prisms.
+ Dark Templar usage in all matchups. Use them. It's a (potentially) permanently cloaked unit. Why did it take until Zest for them to be utilized more? They're a broken unit early and late game, and awesome in many base trade scenarios. Just get a Shrine every reasonably-long game.
+ Big-eco play styles. Why don't more Ps have 80+ probes in long games? Giant economy -> massive bank -> sneak in a warp prism -> "Zerg" the Hell outta Ts and Zs -> "Look ma, meza winner!"