To improve your game in StarCraft 2, you must understand a theory
called Opportunity Cost. It's a little term in economics that
means in order to choose one choice, you may pay a cost in time,
resources, or other options that could have been taken
instead. Abilities, production, and even build orders have
Opportunity Cost associated with them. Back in the glory days
of RTS gaming, you had Age of Empires. There were two terms
for your build order--it was a rush, or it was a boom. A rush
sacrificed workers for early offense, and a boom sacrificed early
defense for mass production of workers to reach a powerful end game
that much quicker. Understanding that basic concept of
decision making is essential to understanding the more subtle ones.
Terran ThemesOrbital Command upgrade speed - The faster you get your OC up, the sooner you can start pumping out the lovely MULEs to put your economy into overdrive. Timing your build order to have an SCV finish as your Barracks finishes, and then having at least 200 minerals on hand (for a Marine or Tech Lab, and the upgrade) is a key part of refining your early game build order. This goes for creating expansions as well. If you are building your expansion in your base with the intention of flying it to its destination, you also have to consider this. Unless your are moving to an island expansion where you need to carry SCVs with you, upgrading to an Orbital Command before positioning your Command Center can get you half a MULE's worth of energy while en route to its destination. This would also enable SCV production the moment it arrives! Of course, that point is naught when you realize that you have to spend that time creating the Orbital Command either way, but it's much safer to upgrade in your base than in the field.
Orbital Command energy spending - As glorious as the MULE and Scanning are, and lets say the Supply Drop too, each one costs 50 energy in a limited pool. Using one ability limits the ability to use the others. And there is nothing worse than forgetting to bring detection with you and suddenly Dark Templar are cutting up your Siege Tanks. You hit your hot key for your Orbital Command, and instead of hearing Dark Templar explode, you hear Not Enough Energy. While this could be averted by saving some energy for a Scan, saved energy in the Orbital Command is worthless. You could mine 270 minerals faster and free with a MULE, or generate 100 free minerals with a Supply Drop if things are looking grim on your expansions and mineral lines. It's up to you, the player, to walk that fine line of use and conservation for a disaster. There is one general rule of thumb though--a 50 Mineral SCV is almost always better as a scout than a 50 energy scan.
Moving out - When the time is right to move out, you have to make sure the enemy isn't waiting for you to roll out and leave your base defenseless. This goes for any army, but doubly so for Terran due to their slow units. Should be losing a fight and decide to retreat, that often won't be a viable option since your units are so slow and possibly immobile due to surrounding or Siege Mode. So when you move out of your base on the offensive, it must be decisive. It must have the edge to, at the very least, devastate your opponent's army into a retreat. Even in the skies, your units will be outrun. In the end, you need full intelligence on your opponent's forces to know that you have an advantage you can press. Scout early, and scout often so that you know your attacks will be overpowering and require your opponent's full attention, rather than having them try to sneak in a pair of Void Rays while you're in the middle of the field. Your force must be devastating enough that those raiders are necessary to defend!
Zerg ThemesExpansion Timing - Zerg are by far the easiest race to fast expand with, and that fast expansion can give you the early production and resource edge you need to win. However, fast expansions are also highly vulnerable to rush and harassment tactics. The timing of your expansion and Spawning Pool sets the tone for the game. If you're unfamiliar with this notation, the number beforehand is your population in workers before you put down the building. These are listed in order of most offensive to least offensive.
10 Pool - Enables punishment of overzealous economy builds while not ruining your own
Overpool (Built after your Overlord)
15 Pool - Hatchery is thrown down immediately before or after it to enable faster double Queen and a boom in production far sooner.
The later your Spawning Pool, the earlier your Hatchery can be put down. If your opponent scouts quickly and sees no Spawning Pool down, they may change their tactics to a Rush oriented strategy. You must have solid unit control to do these early hatchery builds to keep your drones and initial Zerglings alive from the inevitable harassment or rush, until your Queens can assist in driving it off. On the other hand, a powerhouse economy that isn't rushed inevitably leads to an incredibly potent mid and late game--just ask Idra, one of the top Zerg players who almost always does a 15 Pool, 14 hatchery.
Queen Use - Just like the Orbital Command, Queens must use their energy very carefully for the best thing possible in the situation. Spawn Larvae is your bread and butter ability that should be used on a Hatchery every time it's up for the most part, unless there are a few things going on
#1 - Do you have a line of Creep going to your expansion, and a good ways in front of your expansion? Do you have sight all around your base? Intelligence and Speed are as essential as production to a Zerg player.
#2 - Can you afford to produce from all of your bases if you're using Spawn Larvae at each? Sometimes, before a base is saturated with workers, you simply cannot afford to keep up full production and you'll sit at 3 or more larvae, wasting the natural production and as a result wasting the energy of your queen.
#3 - Are you harassing? Saving your energy to heal heavily wounded Mutalisks is like putting another 100 minerals and 100 gas back in your pocket, and if you're continuing Mutalisk production, you likely won't need the Spawn Larvae to keep your gas reserves at zero from production.
Protoss ThemesChronoboost Use - From the start, you've got access to boosting any production or research by 50%. The first Chronoboost you do will set the pace for the rest of the game, much like the Zerg timing of their expansion and Spawning Pool. Many players opt to Chronoboost their Nexus once after their first pylon finishes, and then switch into production Chronoboosting until Warp Gate tech is available, and then Chronoboost that to completion ASAP. Chronoboosting your Nexus more than once can supercharge your economy early, but requires you to put more minerals into Probe production and can delay the timing of your Gateway and Cybernetics Core, and possibly run you into supply cap issues early on. Chronoboosting research out is a mixed bag as well--would your army really benefit more from it than faster production of units? Take into account your current income level and threat of the enemy before your Chronoboost. If you can't afford your current number of Warp Gates, there is no point in Chronoboosting the cooldown on them, which is a solid time to Chronoboost your Forge to complete those valuable weapons upgrades.
Hard Counters vs. Hard Counters - The Robotics units Immortals and Colossi are absolute powerhouses on the battlefield, but also incredibly easy to counter. You have to make a heavy judgement call when you bring these units into the field of whether or not they can serve their purpose before they are annihilated by Zerglings, or anti-air units. Colossi themselves are a massive tech investment of at least 700 Gas before one useful one is in the field (100 Facility, 200 Support Bay, 200 Range Upgrade, 200 for one colossus). As such, hiding your tech is incredibly important so that you have a window of dominance to make something happen before the hard counters to these monsters hit the field and quickly destroy them or at least hinder their use.