Classic RTS gameplay that's as good as it gets.
Single player missions are molded around standard RTS rules and often include rescue, sneaking, survival, reinforcement and intercept objectives. The player has primary and secondary objectives to complete during the mission that typically require you to defend and conquer specific points on the map. Secondary objectives don't have to be completed, but finishing them often helps you with the primary objectives in some form or the primary objectives in future missions (if completing a series of secondary objectives across missions). Between the missions, players can spend research points (gathered during the missions) on upgrades spread across an enhancement tree. Taking one path locks you into the upgrade path for a particular power (like increased armor) or unit (like Goliaths or Firebats). New units can be purchased during a mission and they get tossed onto the battlefield to help you fight off the alien scum. In addition, mercenaries can be hired for an extra cost if the battle is turning in the favor of the enemy.
By design, the single player campaign is created for multiple playthroughs. The campaign typically stretches about 15 to 20 hours, depending on the difficulty. Playing again with entirely different upgrades offers an alternate experience during battles. There are also multiple ways to approach a mission with stealth or direct offensive available to you. While the single player story doesn't change, you can earn achievements which are tied to your Battle.net online profile. These achievements are broadcast over the network to your friends when achieved. Blizzard is really pursing a platform of "online all the time" design with Battle.net. You have to be online and logged in to earn achievements as well as get blasted with requests / achievements of your friends (very similar to Xbox Live).
Connecting through Battle.net is pretty simple as well functionality to search your Facebook friends and add anyone on your Facebook account with a Battle.net account to your friends list. Of course, you may be adding old high school buddies that you haven't talked to in 15 years, but it can be helpful in jumpstarting your friends list for online play. Speaking of, all races in the single player campaign are opened up in online multiplayer. Each race has their strengths and weaknesses as well as strategies that can be applied during a match to become the dominant race. Many maps support up to 12 players and players can be grouped in teams as well; all AI controlled teams can be created as well if you want to team up with friends against difficult AI.
If you assemble a consistently well-oiled machine for a team, there are also league matches; ranked for performance with leaderboards. During my matches, I heard plenty of people complaining about the lack of a LAN mode for private games, but the Battle.net experience really isn't that bad. Laggy players can be booted from matches and there are tools in place to identify those people. Overall, the multiplayer is very much designed like the original Starcraft and Blizzard did a great job of capturing the original feel and addictiveness of the first title in the series.
If you are into real time strategy in any form, it's hard to ignore Starcraft II. The exceptional single player campaign is nearly worth the purchase price, but adding the extensive multiplayer component into the mix really ups the replay value of this title. It's the type of game that you can jump into a couple years from now and still find a thriving online community, something that the console community doesn't get to experience often. My only caveat is that the similarities to the original Starcraft may turn you off if already tired of playing the original over the years. That being said, this is highly recommended for RTS fanatics or those who enjoy sci-fi themed games.