I would beg to differ. The reason why this game has been so widely successful for so long is that the different races were quite different while remaining quite balanced. It's the first and only RTS that I've ever played that went beyond just giving the same units different skins, and maybe an extra bonus "unique" unit. While it is true that it is probably easiest to win with the zerg, that is only because you can "rush" with them. However, if you use the correct tactics, the terrans can easily fend that off, and the protoss, for that matter. A dozen zealots can hold off a horde of zerglings, even a few dozen hydralisks. So, to make it short, with some knowledge and experience, this game is easily one of the most balanced, tactical and replayable games probably ever created.