Gameplay, Gameplay, Gameplay
What else is there to say about Starcraft? It's a Blizzard game. I'm not sure why God endowed Blizzard with so much talent, but three game series--Starcraft, Diablo and Warcraft--prove that Blizzard will always have a place in history as one of, if not the, best developers around.
As an archetypical RTS game, Starcraft kind of sets a standard for what a good RTS game should be: the missions are varied--and not too long or short, the maps are varied and look good, the overarching story is believable and comes together well, there are distinct sides you get to play which adds to replayability, a memorable cast of characters, there is a wide variety of units that all have a tactical purpose and can give you many ways to achieve victory, these units look cool, the resource gathering is simple and doesn't take too long, buildings can fly (had to add that), the interface is simple/easy to look at and control/provides all the necessary information and finally, well, wait I think I just covered everything in this massive paragraph.
The only weakness of this game that I can see is that due to it's age, the screen resolution is kind of low and the graphics aren't going to compete with, say, The Witcher.
But who cares? It's addictive, fun, and like Diablo, is almost like a Rubrick's Cube as far as longevity goes. Play it!