- crisp, clear, appropriate, and useful sound effects
- three playable factions -- far more interesting than one vs. one
- music that accurately fits the mood of play for each faction
- factions have very distinct capabilities (so much so that the word "zerg" is now a commonly used verb in gamer circles)
- superbly balanced gameplay overall
- gorgeous translucent UI
- effective unit controls
- human "Ghost" units can drop nukes on opponents -- lovely!
- evocative voice acting (much better than the usual)
- an actual story that you can care about
- a story with characters that you can care about
- a love story that you can care about, of all things!
And a final point, just to prove that I'm no mere fanboi: visit any electronics store that sells games, and you will find Starcraft still on the shelves. Realize that retail shelf space is so valuable that most games are gone in four weeks, and that Starcraft originally shipped in 1998, and you can only conclude that this is one of the highest quality PC games of all time.
Having said that, I find I'm not much interested in the sequel. It appears that what I enjoyed in the original, which was the nice balance of planning and action, will be shifted much more toward the action side of the ledger (perhaps to satisfy the Korean gamer market). From the previews, Starcraft 2 looks more than anything like a cross between Starcraft and God of War, all hyperemotive ("CRUSH YOUR ENEMIES!") and particle effects.
But maybe I speak too soon, and there actually is a compelling literary experience underneath all the flash.