Hey everyone! First of all, I'd like to apologize for not posting for a while. I started playing this game called minecraft and it totally engulfed my weekend. @.@ By now though, I think I can handle myself well enough to be able to play during my leisure time and get other things done.
Now that that's taken care of, let's continue with the topic at hand.
So like I was saying before, building placement really is key, and is something often overlooked by newer or less experienced players. If you watch more experienced players, usually building placement is key, especially for zerg and protoss since they don't have that easy wall in. Many players may see it as sort of a cheese or lame tactic to use your buildings to block off your ramp or workers or something, but it is far from it. It is cheese to early mass marines and go slaughter someone, but it's not cheese to use strategy and think of where to build your key structures.
In this section, like I said, I'll go more in depth and tell you exactly what it means to place structures strategically.
Terran players have the easiest time with structure placement as they can very easily hit the "redo" key and lift and replace. This brings up some key points though. With terran, you can place your first barracks in a good location, then build a tech lab/reactor and later on as soon as your first factory/starport is done, move it over to that first addon and start building later tier units much faster. In case you couldn't figure it out, this is how many players are able to get that quick banshee rush or tank up. Also, it's important to note that you can build and lift your command center. Many players build the CC initially behind their wall in and pump extra scvs to bolster their economy. Then lift it into the natural expo with protection.
Protoss players have a bit tougher of a time with building placement, but have a lot of possibilities like terran players since they are only limited to the pylon power. For the initial choke point, what is very common (and works very well) is to build your pylon a bit behind your ramp then place the gateway in front. Now this can go one of two ways: first is to build a second geteway next to it leaving only a 1 square space. This is for general 2gating and helps to choke in your base as well. Another idea is to put up your cybernetics core next to it leaving again, one square.
Both the gateway and core are very strong and are much too heavy of structures for early units to take on. But what do you do with that little hole? You fill it in with a single zealot on hold position. This makes an impassable wall until the buildings or your zealot is killed. A good follow up is to have some more zealots in reserve, or better yet, put a sentry near so you can easily use a forcefield and block the ramp entirely.
Another interesting thing about protoss is that you can build structures anywhere and easily walk away. This is especially useful for hiding buildings like a quick dark shrine or stargate. This is also useful for placing proxy pylons near your enemy. As scouting is very key, this can completely throw your enemy off guard if he thinks you're going for a land based army some phoenixes appear out of nowhere.
And finally that brings us to the zerg. The zerg have arguably the toughest time with building placement, yet they also have a lot of interesting options open to them that the average player would not initially think of. In the beginning game, I seem to notice that most novice players either put their pool in a completely random location, or hide it behind the hatchery. This may seem like a good idea at first as you need to protect it, but it can in fact be detrimental. If the pool is placed alongside the gas and hatchery, it blocks off easy access to your workers and a quick rush in that is preventable by guarding the other side. Granted, it's not too common to run into a very early rush, but it does help. Also, when placing the second building, whether it be a roach nest, baneling nest, or even an evo chamber, it is wise to place it on the other side of your workers to really protect the mineral line.
Next what you have to think about is the creep limitation. Obviously, the zerg can only build on creep (except the hatchery). This becomes key as it forces you to spread creep more evenly and quickly. This is all very possible with overlord creep drop as well as queens pooping out those creep tumors. What you might not expect, but is a good plan in a pinch, is to move your overlord to a place away from your main then placing a quick spire underneath its creep. Like the protoss move this keeps your opponents in the dark if they scanner sweep your base.
One more specific point I want to make about the zerg is their ability to move the spine/spore crawlers. This is a very important thing to realize in the early game. As you can build some spines to stop an early rush, you can then move them out into your natural expansion as some early defense. Don't forget about spine crawlers in your main if your natural is now your main line of defense! With overlord movement upgraded you can bring in overlords to drop creep and even advance with your spine crawlers and keep your opponent in his base.
So to wrap things up, you just have to keep in mind how important buildings are and building placement is. They are more than just the places you need to build better units. They can easily be utilized to help with base defenses or even trick/harass your opponents. It's worth it to experiment with different implementations of your buildings and placing them in different locations.