Easy to learn, hard to master, big and open enough that 100s of different strategies, tactics, and play styles have been created over the years. Xplore, Xpand, Xploit, Xterminate!
Trailblazing Tutorial mode, extensive tech tree, ample freedom for race and ship design, smart enough AI players to make solo play a challenge.
But multiplayer sets apart the toughest strategists from mere dictators in scenarios where less than 500 worlds to master is considered "small". Diplomacy, management, trade, stealth, dirty tricks, tons of shiny ships turned to smithereens. You could lose 100+ big battles, turnaround and win 200+ more before defeating the last enemy.
Shout "Packet!" in a room full of gamers, and Stars! veterans cannot help but shiver. P-}