One of the best RTS games ever.
This game stuck, in many ways, to the different units and buildings from the original Red Alert, but managed to add a lot of unique aspects without getting stupid. First of all, the environment (buildings, civilians, etc.) was actually important and useful for once. It was extremely cool to be able to make a viable defense of an area just by barricading a bunch of buildings, and you could mind control anything in the game (virtually). They also threw in strategic civilian buildings that ranged from mildly helpful to game changing.
Multiplayer and skirmishes added many great elements to the game, including country specific units, etc. But one of the REALLY cool things about this game was all the little hidden bonuses that took a while to find. If you captured or infiltrated certain combinations of buildings of other countries using spies/engineers, you started seeing some prettttyyyy interesting units start to appear in your building options. Teleporting Ivan unit that can plant bombs on things? Hell yeah.
I would also mention that both the campaign missions and the skirmish maps were very inspired. There were a lot of "real" locations used, and it was fun blowing up the Pentagon as the Russians, or staging a defense around it as the U.S. The skirmish maps were (for the most part) interesting, with the civilian buildings being placed intelligently and life was good.