A time ago, it was rumored that you in Tiberian Sun, would move your armies on a global map and then zoom in for battles. Tiberian Sun will use the same system as in the original C&C: Tiberian Dawn and C&C: Red Alert. Changing something that's so great already makes no sense.
Between each mission, there will be cut-scenes as in Red Alert followed by the mission briefing. Unlike Red Alert, Tiberian Sun won't have "Talking Head briefings", instead the player's character will appear in the movies, giving a much stronger connection to the plot. For the actors from the previous games, we don't know if they'll return, but it's most likely that we'll see Kane again. Picking a certain mission will effect how other missions are played and EVA will tell you what happened while you were gone. You will be able to choose your first units before the missions start.
There'll be two playable sides, GDI and NOD. Although it was (or is) rumored that the third side called The Forgotten (originally known as the Freaks) will be playable too, I doubt so. It's possible of course that you in some mission maybe are supposed to rescue a few units captured by NOD and that you might start some missions with some of their units, but that they'll be a complete third side with their own campaign, I really don't think. However, according to some sources, The Forgotten will sometimes agree to ally with you, which will probably give you access to their units, buildings and techs.
As there were special characters in the other two games, I can guarantee that we can expect the same thing in Tiberian Sun. Although several special units have been rumored, Westwood have not confirmed anything yet.
There will be a map editor included with many more features than the one with Red Alert. You'll able to create your own campaigns with units, triggers, structurers etc. I also think you can expect Westwood to include some kind of a unit editor, where you'll be able to modify the existing units and possibly to create your own. However, the map editor will not be included on the original CD. Westwood will relase a mission disk, which will include the editor. Erik Yeo said that the map editor for the WW-developers was too uncomfortable :)
Before releasing a game, everyone makes a beta of it, and Westwood will definitely make one for Tiberian Sun too. However, if they'll let the fans test it as they did with Sole Survivor is still unknown, but to be sure as many bugs as possible are fixed, I think you can expect a beta sign-up on their site before the release. They'll probably be making a demo, as they did so for the other two games (however, the Red Alert demo came about a year after the release). This time, I think you can expect a demo right before or a few weeks after the release as it'll most likely make more people to get the game.
As confirmed by Erik Yeo, there will be no support for 3D Cards in Tiberian Sun. Using their own technology called Voxel which is like a 3D-pixel (will give the game a more 3d-ish look), it would be unnecessary to add 3D-support.
In some missions you will be able to destroy the enemies in base light, and then attack the base with the dark as cover. Under one missions you will have to get into your enemy's base and hack their defense system so you can secure your mainforce sweep.