Great post giving a good rebuttal on why you should actually play Tiberian Sun, and an especially good summary on how it too pushed the boundaries of the campaign format. Bookmarked for a future thread index update.The importance of Tiberian Sun is that it single-handedly shows that the RTS genre can be atmospheric and evoke feelings of loneliness, helplessness, and other emotions typically associated with first person games or games that are more about the story of a single character. Tiberian Sun, especially compared to CNC3, did an extremely good job of making the world feel like a barren, barely habitable, treacherous place, filled with hostile life and a toxic atmosphere. CNC3 feels very clean, even in the "Red zones" and virtually every other RTS game I have played that's set in a post-apocalyptic or terrifying, infested setting (StarCraft zerg worlds, Total Annihilaton decimated worlds, Dawn of War endless war etc) just comes across as having a slightly beaten up map that never feels truly inhospitable or dangerous. Most of these places just feel like "game maps", if that makes sense?
I wholeheartedly disagree with you that Tiberian Sun's gameplay is essentially a tech demo, and I feel like you're selling the gameplay short. Great thought and care was put into making the factions different enough to be relevant - GDI has an emphasis on big, clunky land units (Titans, Mammoth MK II, etc), while NOD is more infantry and stealth focused, and this design isn't just reflected in their special unique/commando units, but in other more minor faction differences too (GDI has a vehicle transport Orca, while NOD has an infantry helicopter, for instance, and in Firestorm this is extended to both the GDI mobile war factory and the NOD mobile barracks). Terrain deformation as well as elevation is interesting in regards to certain aspects like disk throwers projectiles having physics, however it is admittedly undercooked. TS's gameplay is far from super innovative, but what's there is there and is interesting.
One of the innovations you didn't mention is how doing optional, hard side missions before main missions can affect what units and resources you have access to in the main missions. For example, in one case you have an optional mission requiring you to capture various bridges, which is a hard mission. In the main mission alongside it, the bridge will explode and prevent enemy reinforcements if you completed the bridge capture mission. I believe this was also removed in Red Alert 2, along with terrain deformation and other things.
Where this aspect really shines is in the "gimmick" missions. Every C&C game has them, you know the ones I'm talking about. Usually you start without a base, often with a special commando or unique unit, and have to infiltrate or take out a special objective, or do something to that effect. The Hospital GDI mission is amazing, and NOD has a few too, where the unit design of the special units aids the gameplay significantly. In the hospital mission, for instance, you have a relatively weak mutant which can instant-kill infantry, a slow firing railgunner who can take out light vehicles, and a defenceless hijacker. Using all three of them together is important and forms most of the challenge for the mission.
I would recommend giving Tiberian Sun another playthrough if you have a chance, and really let yourself get engrossed by it's world. You will have a great time. You should play the expansion (Firestorm) too, which is excellent.