What makes C&C "C&C", by FRAYDO.
1) Immersive universe. Tiberian Dawn had an amazing balance of the real-life warfare we experienced at that time and science fiction introducing the alien substance that is Tiberium. Not only was Tiberium this resource that had to be collected in order to finance your war machine, it was the focal point of the entire world. This quasi-religious organization sought to use it not only for financial gain, but for the advancement of technology and humanity. "The Technology of Peace!", as it came to be known by devout followers. Conversely, you then have our equivalent of the UN forces in GDI. The Global Defense Initiative, united in fighting against what they perceived as tyranny and fighting to protect the free world. Despite their noble ideals and intentions, they were not above using Tiberium for their benefit. Tiberian Dawn, and later Tiberian Sun, potentially had you conflicted. Is Nod's vision for the future one that should be pursued? Is their way of evolving and elevating humanity the route we should be taking? What of GDI? Are they halting the progress of mankind in stopping Nod? Or is GDI the best hope for Earth, in their mission to eliminate Tiberium and return to the status quo? As the series progresses, it becomes evident too that GDI was quickly becoming too powerful to be restrained, eventually becoming their own superpower and no longer operating within the UN's jurisdiction. Was Kane correct in warning the world that if GDI's strength was left unchecked, they would rule the world with an iron fist? Between playing the game and watching the storyline unfold in the cutscenes, it's these questions you ask yourself that immerses you further in the game. You weren't just clicking a unit here and moving him to over there. You had chosen a side in this conflict between GDI and Nod, and you were fighting to see their vision through to the end.
2) Memorable soundtrack. We all know of Frank Klepacki, the legend. He recognized that music is all too important in story-telling to be ignored. He may not have realized it at the time, or maybe he did, but his contributions in that front is what brought the game to life. You can add in a fantastic story, an impressive list of units and vehicles to fight with, and even A-list celebrities, but in order for the player to experience all this they must actually play the game. When Act on Instinct hits when you enter your first Tiberian Dawn mission, you're immediately engrossed in the mission. Get moving, soldier, we have a war to win. Better put, "We are going to have to act, if we want to live in a different world." After that song ends and the shuffle kicks in, you're still engaged in the game. Each track playing was catchy enough yet not distracting, playing on a delicate balance of keeping you the player engaged but not driven away by the music and still allowing you to focus on the action at hand.
Before anyone says "if Frank Klepacki didn't make the music then it's a terrible C&C track", let me just state that you are wrong. Jarrid Mendelson did an amazing job on the Tiberian Sun soundtrack, both on the songs he did in collaboration with Klepacki and those he did entirely of his own.
3) "Classic" RTS gameplay. Not much to say that the others have not already. Deploy the Construction Yard. Build the Power Plant, then the Barracks, then Refinery, collect money, build your army, find the enemy's base, send your army, win. Command & Conquer ™ . Any variance of this (Generals, Red Alert 3, and C&C 4), is initially met with resistance. Generals and its Bulldozers/Workers system I came to like, Red Alert 3's style of Allied Prospectors and Imperial Nanocores I could barely tolerate, and C&C 4, well just no.
4) Idk. I just love Command & Conquer. The series has its ups and downs, but as a whole I love everything the series has given to me. The good memories, the communities it has brought me to, and my passion for blasting the C&C soundtracks on my car radio.