As a long-time high level SC2 player, one additional thing that makes SC2 so difficult is that the game has multiple layers of tactics and strategy that require specialized logic, but those layers also interact and synergize in a deep way.
- There is the overall strategic game of 'Who is ahead economically? Given that, should I be expanding, attacking, or defending?', with the implicit understanding that the player with the current economic advantage puts pressure on its opponent to attack
- There is a resource management and build-order system where you need to plan and optimize building as big and as effective a unit composition as quickly as possible, except there are a lot of tradeoffs: you can build for a stronger army sooner, as opposed to a weaker army alter
- There is a tactical micromanagement battle where small groups of units are pitted against one another, and where small tactical movements can gain very large materiel advantages. Units are relatively short ranged, so to damage or defend effectively requires effective positioning. Most armies fight better as a cohesive group ('ball'), except there are units that specifically punish and do splash damage that need individual micromanagement. Battles can take place over a short period and be over quickly, or can be long-running positional skirmishes that last for half the game, where each player is constantly probing for weakness before one finally goes for the throat.
- The economy fundamentally depends on worker units that are vulnerable to harassment, so the tactical battle requires a choice between putting everything into one large army and pushing, or splitting units into smaller groups and harassing in multiple places, or various mixes (small group to harass, bulk of army to defend, etc.)
- If keyboard and mouse action rates are capped, then at every moment in time, the player must decide whether it is more profitable to devote actions to managing the army (micro) or managing the overall economy (macro). Choosing wrongly usually results in a loss
- There is an implicit rock-paper-scissor tradeoff at the highest levels of the game: a 'greedy' strategy that cuts corners and favors economy over military will generally beat a 'safe' balanced strategy. Very aggressive strategies win against greed and generally lose against safe
- There is the ability to scout your opponent to see whether they are going greedy, safe, or aggressive, but scouting requires an early investment in units and making subtle inferences about the opponent's build order, so the choice of whether to scout and how is not a trivial one
- There can be bluffs where your opponent purposefully allows a scout of a key building, kills your scout, then cancels that building and chooses an entirely different technology instead
And all these layers interact:
- For example, if you go for an aggressive strategy, then you must commit blindly at the beginning of the game and often try to deny enemy attempts to scout you
- If you scout that your opponent's army consists of units that are faster than yours, then they generally have much higher harassment potential, which pushes you towards a defensive posture. On the flip side, your opponent can use this threat to improve their economic position instead of attacking.
There is long-term planning at the strategic, informational, and also tactical levels. Effective high-level play requires an accurate model of what your opponent is doing in an environment where it's easy for your opponent to deny acquiring that information.
I'd wager that if you took two evenly matched professional level players, and then revealed the entire map to one player but not the other, you would go from a 50% to a 95%+ win rate.