I'm very strongly interested in this aspect, indeed it is one of the key reasons I backed this game. The potential is there for this game to surpass the original design of SWG, which in the end became a "place pre-made structures only"system.
A key aspect that will need to be tested is maintenance costs. IN SWG my experience was that even with a busy player city, being an architect in SWG was a lot of fun but ultimately the cost of maintaining buildings did not justify the build + maintenance costs. A simple house next to the 'default capital city' on a server was a better location to craft and sell things than anywhere else, and did not have the huge up-front time and resource costs either.
That aside, the last game which offered anything like the building flexibility the videos have shown is intended here was venerable old Ultima Online. I spent years tweaking and building houses there, so I have absolutely no doubt that if they implement a good system, I am going to love it and push it to its full potential.
Fun times ahead... although it will be interesting to see how they handle loading times, because if someone like me creates a town (or heaven forbid a city), there might be a whole new level to the phrase "player dungeon". Fun :)