A large part of the success of Red Alert 2 comes from its smooth control system, comfortable menus and intuitive balancing.
Granted, it's a formula but it's a formula because it works and only Starcraft and Age of Empires have done it as well. The game is still about harvesting resources to build structures to buy units, but what seems tired and worn-out in other games is inspired in this game.
But what Red Alert 2 is, is one of the finest explorations of what can be done without taking the genre beyond its present limitations. This is not the 'next big thing' in strategy games and it certainly won't have the impact on the genre that early Westwood titles did. Having said that I should caution you that Red Alert 2 does not offer the same kind of revolution in basic game design that so many of us expected of Westwood's last sequel, Tiberian Sun.
There's an amazing depth to the tactics of the game, a good range of missions in the single player campaign and a kind of slightly ridiculous aspect to the story that lets you now that the game is supposed to be fun. Red Alert 2 is one of the most polished, best-conceived, and downright enjoyable real-time strategy games in recent memory.