Once you reach that stage, it's all about getting an economic edge by cutting corners or knowing when to cease production, and being able to minimise costs.

At that level it gets really subtle. A great player will use the exact same build order as a good player, yet everything comes out faster, just because he's minimised the time where a worker was in transit, or because the pylon dropped at exactly 100 minerals, and finished just in time to prevent the player from being supply blocked (rather than too early, where you've essentially wasted minerals that could be spent elsewhere, or too late, where you've blocked your own production).

Great players always look like they're about to lose, right until they win. They can produce the bare minimum of troops necessary to survive, injecting the leftover resources a lesser player would have spent on defences into economy or tech instead.

It's a common mistake to focus on micro when you really should be focusing on macro - trying to save one or two units when you really should simple be producing twelve more instead.

Great players are also good with their timings - knowing when they are vulnerable, and when their enemies are vulnerable.

But that's all pretty subtle stuff, much beyond my skill level, certainly.