The Tiberian Sun demo features two tutorial-like GDI missions that are not included in the full version of the game.
The Tiberian Sun Demo, while not exactly new, is still just as fun as the full version of Tiberian Sun! Within are two special missions which are not featured within the standard full version campaigns. Overall, it's a good download, and fun to play, even if you own the full version already, because of the fun new missions. Better late then never!