StarCraft didn't become the king of real-time strategy games with flashy gimmicks and graphics. This is a deep game that awards intelligence and experience (unlike Wii Sports), requiring careful resource management and positioning of units. Any unit in your army is vulnerable to something and you need to keep them away from that while maximizing your strengths. And unlike Warcraft II's Orcs and humans StarCraft's Terrans, Protoss and Zerg had much more than cosmetic differences from one another.
The Terrans are human exiles from Earth colonizing distant planets. They emphasize the defense and mobility, able to build solid bunkers and tanks that transform into artillery cannons. The buildings can lift off into the air and fly to another location and their buildings and mechanical units can be repaired.
The Protoss are suspiciously hominid aliens with advanced technology and psychic powers. The Protoss place a bigger investment in each unit then the other two races. While there've units are generally more powerful they also cost more resources and take longer to build. They also construct buildings by opening warp gates so they can build with minimal effects on gathering resources (the Terrans need a worker to stay with a building under construction).
The Zerg are mutant space bugs who consume and assimilate other lifeforms into the Swarm. The Zerg build units quickly favoring quantity over quality. They are infamous for quickly building a small force and attacking at the start of the game. so infamous that the term “Zerg rush” or “Zerging” has become a common noun in video games meaning “Using massive amounts of weak, or generic units, to attack an enemy.” (quote taken from urban dictionary).
That's the basic gist of StarCraft's three races. There's much more to them than I'm saying here each of them could account for their own game in the 90s and yet each of them are very finely balanced. Many games have been dominated by certain play styles or simple strategies. But StarCraft's design never faltered even under the strain of being played at a professional level were many games fall apart. (Just look at Super Smash Brothers)
It also helped that StarCraft was also a competent space opera taking inspiration from a wide range of popular science fiction movies. From the Terrans alien like blue-collar life too the Zerg's resemblance to the bugs from starship troopers and aliens, well, alien. The protons even find room for Star Wars like mysticism. StarCraft might just embody everything fund about science-fiction.
It might also have helped that this game was very undemanding of hardware and could run on the older and cheaper computers of the time. And that blizzard continue to support their game even a decade after release (going as far as to patch their Mac version for any platform migration, like OS 9 to OS X.)
As I write this it is the eve of StarCraft II the sequel everyone has wanted for about the past decade. I'm overwhelmed with hype and nerdy glee (as much as I can be anyway I'm about as excitable as a cat). My hype for StarCraft II is only matched by BlazBlue: Continuum Shift which is being released the same day for some strange reason.
I'm not sure if even its sequel can end StarCraft. Somehow I'm sure that some will make a “StarCraft classic” mod which would allow the game to continue his immortality in a better interface.