Environment interaction and AI are really surprising, but what a pity there are only 2 factions
AI of the game can prove quite outstanding if you have played the Campaign and Skirmish. First off, have a look at the way AI opponent struggle for resource. They defend the mines quite well with enough number of troops to cause you trouble and even construct extra defensive building on important locations. Even if you can gain control over some mines, you will find that machine force is always ready to re-occupy them if they are ill-defended. On battlefield, AI also exploits the environment to save their units and makes decision to annihilate or retreat according to the number of troops you have. More amazing is its ability to combine power of all arms: Infantry, Airborne and Armor Company (look like AI can "micro" units).
CoH has many other good points. Graphics and sound effect of the game are truly uncriticizable. Besides, the game is also very balanced between different modes, since Multiplayer is as good as Campaign and Skirmish. There is only one thing that disappoints me: the number of factions. Two is probably too few for any RTS nowadays. What a pity developers of Relic did not repeat what they have executed quite well in the earlier "Dawn of War".