Where you get to deploy your forces in a turn-based strategy versus three Alien races prior to battle - Seems much more sophisticated than Command and Conquer, as it forces you to think strategically rather than mass-produce units; being said, my favorite unit is the Commando, followed by the upgraded Elite trooper - Grenade troops are overrated. Each alien you kill is enjoyable, and any cheap tactics used by the enemy, makes you want to avenge your fallen comrades. When you are placed on the defense, it realistically represents the challenges of defending against an determined invading force that has not a lack of surprises.
At a certain point in the game, you are allowed access to alien technologies (by capturing research stations) to further develop your forces. Sadly, the hover-research is not useful, as the units are too weak for combat.
Also, the music in the series is ubiquitous and well-known to fans of the series, and represents humanity's victory over hostile and technologically superior alien forces, hence the name, Liberation Day.