The great wall is an actual brick and mortar object, and the UN an actual building (a very nice one actually) that houses a sadly inept world governance organization. The "concept" the UN stands for could be represented in a dozen ways; the League of Nations, the Hague, Rome, even the EU.
Looking through the wonders, the only one that doesn't seem to fit this pattern is rock and roll. All the other wonders are physical objects, however none of the projects are.
Um, the UN is most definitely *not* a building. There are a few buildings that house parts of the UN (some in NY, some in Vienna, some in Geneva), but the UN itself is an international organization. That's like saying that Congress and the Capitol Building are the same thing.
Hollywood is only a brick and mortar object in the same way that St. Louis or Duluth is. It's not in the game as a wonder because of it's physicality; it's in there because of the cultural objects it produces.
You could say that the Internet is a physical object (in the form of several million miles of copper wiring, switches, routers, etc.) just as easily. The Manhattan Project likewise resulted in physical objects: Fat Man and Little Boy.
Besides if they are just for balance, why put projects in at all?
Because they all perform necessary and/or interesting game functions. However, the ability to rush any of them opens the door to some pretty obvious exploits, so they have to distinguish them from wonders. It would make 1-tech advantages absolutely game breaking and narrow down feasible choices for the end game (the ability to rush the internet, for instance, would mean that Fiber Optics should be beelined in nearly every game). The designers wanted the modern age to have just as many meaningful choices as the Ancient Age. I don't think they fully pulled it off (in practice, you're usually locked into 1 or maybe 2 possible victory types by this point anyway), but I can definitely understand why they needed some wonders to be unrushable.
The only one I don't really agree with as a project (from a balance standpoint) is the Apollo Program; as the long as the actual SS parts remained unrushable I can't really see any major abuse potential. The only time I can see it even making a significant difference is if someone rushes for Fiber Optics (for the Internet) while ignoring rocketry -- rushing the Apollo Program would allow them to catch up in the SS much quicker than usual. But it's a pretty unique set of circumstances that makes this gambit worthwhile (extremely high production, less research than at least two opponents but enough research to beat them to Fiber Optics by beelining, and some condition that makes conquest/domination less desirable) so I can't imagine it would alter the game all that much.
