According to its creators, StarCraft II is designed to be the "ultimate competitive real-time strategy game", building on the successes enjoyed by its predecessor, StarCraft.It features the return of the three races from the original game—Protoss, Terran, and Zerg; Blizzard states these are the only playable races in the game. StarCraft II is also designed to focus more heavily on the multiplayer aspect, when compared to the original StarCraft. The changes include overall improvement in Battle.net, a new competitive "ladder" system for ranked games and new matchmaking mechanics—designed to "match-up" players of equal skill levels. In addition, the replay function, which allows players to record and review past games, is being improved. Blizzard has also stated they have made some changes to the game that were suggested by fans.

StarCraft II continues its predecessor's use of pre-rendered cinematic cut scenes to advance the plot while also improving the quality of in-game cut scenes within the levels themselves, which are rendered on-the-fly using the same game engine as the graphics in the game proper. Blizzard states that with the new graphics engine that StarCraft II uses to render the gameplay, they "can actually create in-game cut-scenes of near-cinematic quality".

Most Protoss and Terran units, and some Zerg units, have been shown on the StarCraft II official website, and in several video demonstrations held by Blizzard.Improvements include advanced scenery allocation and more detailed space terrain, such as floating space platforms with planets and asteroids in the background. Small cliffs, extensions, and even advertising signs were also shown to have been improved and refined.
The new Terran briefing system allows the player to explore the inside of the battlecruiser Hyperion.

The single-player aspect of StarCraft II has also been altered substantially from the original game. The Terran campaign shown at BlizzCon 2007 replaced the original StarCraft briefing room with an interactive version of the battlecruiser Hyperion, with Jim Raynor, now a bitter and hard-drinking mercenary captain, as the central character. In a departure from previous Blizzard games, the campaign is non-linear, with Raynor taking jobs for money and using that money to buy additional units and upgrades. Although each playthrough will vary, the end result will remain consistent keeping the storyline linear. Vice president Rob Pardo has stressed that each campaign will function very differently. The Terran campaign, Wings of Liberty, will place players in a mercenary style campaign, as Terran rebel Jim Raynor performs missions for cash. The second release, the Zerg campaign Heart of the Swarm, will have RPG elements. The player will level up the Queen of Blades, Kerrigan, throughout the missions. In the last expansion, the Protoss campaign Legacy of the Void, the dark templar Zeratul will have to employ diplomacy between Protoss tribes to acquire units and technologies for each mission. Each campaign should span 26-30 missions.

Lead Designer Dustin Browder has discussed with Shacknews some of the unique missions that may be included in the Wings of Liberty campaign. In one level, lava floods the battlefield every five minutes, forcing the player to move their units to high ground or watch their units get destroyed. In another mission, enemy units will only attack the player at night. In another mission, the player tries to influence the tide of an AI controlled battle with only a single unit, a Ghost. The single player missions will be highly customizable. In between missions, players can choose units, buildings and upgrades that are not available in the multiplayer missions.

Galaxy Editor[citation needed], replaces the StarEdit as StarCraft II's campaign editor. It will be more sophisticated than StarCraft's StarEdit and WarCraft III's World Editor for creating custom maps and mods. Units from the original StarCraft not in the multiplayer version of StarCraft II, along with units and abilities that were scrapped during the development process, will be available in the editor. Chris Sigaty, Lead Producer, has also stated that the editor will give players the ability to create RPG, Hero-type units and structures resembling those from WarCraft III. At Blizzcon 2009, Blizzard demonstrated a build of the Galaxy Editor showcasing its capabilities, such as the ability to customize the User-Interface to include features such as the Item system from Warcraft III, Blizzard also commented that it would like to include a Third-Person style perspective for missions though it may not be in the final build.
[edit] Units

StarCraft II: Wings of Liberty features approximately the same number of units as the original game. Some units from the original game are returning, some featuring new upgrades and abilities. For example, the Protoss zealot, a melee unit from the original game, now has the ability to dash forward and quickly reach nearby enemies as a refinement of their speed upgrade from the original. Other units have been replaced or removed entirely. Other changes to unit design have been inspired by story events in StarCraft and its expansion, Brood War, replacing old units with new or renamed versions which sport different attributes and abilities.

Units in Starcraft II have new abilities, compared to the original, that encourage more complex interaction with the game environment. Among these are the inclusion of units that can traverse varying levels of terrain, or have the ability to teleport short distances.Some Protoss units can be warped in to pylon-powered areas using the Warp Gate, a slight modification of the existing building called the Gateway. Starcraft II's campaign will also have exclusive units which would only be playable in the campaign and not in the multiplayer modes of the game. These mostly consist of units which have been scrapped from development such as the Terran Cobra as well as various returning units from the original Starcraft such as the Terran Goliath, Wraith and Vulture.
StarCraft II Marketplace

A major new addition to the map-making community is going to be the StarCraft II Marketplace where high quality maps will be sold for a small fee as "premium maps" over Battle.net. The mode of payment has not yet been announced.

Dustin Browder has mentioned that maps like player-created DotA (Defense of the Ancients) in Warcraft III would meet the quality requirements to be branded as a premium map.