1. Quantity vs. Quality

As opposed to Warcraft III where the player controls a few units with many different special abilities, Starcraft 2 will be a numbers game:

    Starcraft 2 [units] will instead act as "movers and shooters"-mostly autonomous forces that generally lack special abilities, but will instead be used in large control groups to "do their own thing" in battle, rather than requiring the micromanagement of high-level Warcraft III play.

2. More emphasis on economic activity

Another comparison to Warcraft III which had relatively little emphasis on the economy, so players can focus on battles instead. Starcraft 2 will have its pronounced emphasis on bases and expansions as a means of achieving victory.

3. Rushing - strongly encouraged

Unlike Warcraft which introduced neutral creeps to keep the players occupied, Starcraft 2 will put more emphasis on rushing. Engaging the enemy early and keeping him occupied will distinguish good players from mediocre ones. Which leads us to...

4. The good, the bad, and the GOSU

According to Pardo, Warcraft III was more forgiving to beginners (and lamers!). In Starcraft 2, the gap in the level will be a lot more evident. Pro players will be able to mow down their opponents in record time by way of builds, multitasking, and quick expansions:

    Starcraft 2 ... will appeal to hardcore fans by being geared toward "highly competitive, skill-based gameplay," as well as by efforts to "continue to drive new features onto Battle.net".

5. Beyond rock-paper-scissors

Starcraft 2 units will introduce more and more subtle ways to counter the opponent's strategy. The key to good counters will be timing, maneuverability, and position (there's a brand new implementation of higher ground - more damage, fog of war).

    The protoss phoenix, a flying unit, has a special "overload" ability that creates a damaging energy field around itself, then renders it immobile and helpless shortly afterward. In a simulated battle between a player with six phoenix units and another player with only four, the player with six phoenixes apparently chokes and uses the overload ability too early, allowing the other player to dodge out of harm's way, then arrange the four phoenixes around the now immobile six in a loose formation and overload the six into oblivion.