Obscure (But Important) Game Mechanics

A.T.T.

This particular gaming quirk is almost unnoticeable in the normal game. The only places you would see this mechanic has a visible effect is in the custom games and really fast attackers like battlecruisers. Concerning the later, have you ever noticed how when a battlecruiser fires on a target, 1-2 attacks strike the ground where the target used to be, but don't actually hit anything? That's an effect of A.T.T, or Attack Travel Time. It's the amount of time it takes for a ranged attack to fly from the attacker to the target. Basically, if you can see the projectile move between the two units, it has A.T.T. This has an extreme effect in custom games like Special Forces because it gives units like the marine an extreme advantage over units like the stalker because the marine doesn't have any wasted shots, while the stalker can fire up to 5 shots at a target that no longer exists. That's a major loss of dps. So remember: when using units with very fast rates of fire, troops that don't have A.T.T. are significantly stronger than those that do.

A short list of units without A.T.T.

1. marines

2. goliaths

3. specters

4. planetary fortress

5. siege tanks (both modes)

6. thor ground attack (not air)

7. auto turrets

8. reapers

9. ghosts

10. diamondbacks

11. archons

12. void rays

13. motherships

14. sentries

15. immortals

16. infested terran

17. queen ground attack (not air)

18. all melee fighters

All other units have at least a little A.T.T.

Worker Balance

If you read the official strategy guide produced by Blizzard, it will say that the ideal number of workers per standard mineral field (set of 8) is 24.  By perusing the charts they used, and watching the workers themselves, I have concluded that this is not entirely accurate. This is because the increased resource production from the 3rd worker on each patch is minimal, and offset by the benefits of stopping worker production early.
For example, let's say that a worker is only capable of harvesting a mineral patch at about 45% efficiency ( I've found that to be almost exactly the right amount, give or take 2%). With two workers, that goes up to 90% efficiency, which is almost maximum mining speed, but not quite. Adding a third worker will pick up that 10% slack, but will waste most of the workers effectiveness unless it is also picking up the slack on the next patch over.
Having 24 workers will give you 800% combined mining rate but waste 280% of your workers power (for a total of 6 wasted workers), which would be much better spent elsewhere. However, If you make only 18 workers, you harvest at 800% efficiency with only 10% waste (which would be spent traveling between patches), and free up 300 minerals and 6 supply points for other purposes, such as making another expansion, rushing, or turtling. You also have other pluses depending on your race.
For Terrans, this change is extra important. You cannot upgrade your command center to an orbital command until all your workers are finished (not technically true, but it helps), and any mineral inefficiencies caused by having fewer workers are easily offset by the mules the orbital command can call down.
For Zerg, this frees up 6 precious larva to make into troops, overlords, and structures.
For Protoss, the faster they get started, the better, and this technique shaves off 1 minute and 42 seconds from their initial build (ignoring chrono boost), a critical speed boost.

I am the only person who uses this logic. You will not find it anywhere else.

255

This odd little number has popped up throughout Starcraft history. In the first game, 255 was the maximum number of upgrades you could give to a unit, and the maximum base armor level, and that's only the stuff that I know about. It applies to the current version of Starcraft in one major way: with the new unit selection system, 255 is the largest number of units that you can control at any one time. You will mostly notice this appear in the custom games, where things like supplies don't get in the way. but if you make an enormous army of Zerglings, you will find out about this pretty quickly. By the way, look around a bit. I think more instances of 255 do exist, and have just not been discovered yet.

I have recently discovered that this number has also shown up in WoW (World of Warcarft) as an early years cap for the number of characters you could have.

The reason behind this number is that Blizzard Programmers used to use 8-bit pieces of data to store most numbers, and the highest number 8 binary units can show is 255 (1+2+4+8+16+32+64+128=255). As their coding got better, this block became less of a problem, which is why I haven't seen it in HoTS.