Internal architecture of Sagrada Familia is as amazing as its external appearance.
As I walked into the church, the first thing amazed me is its ceiling height and the many tall columns - a column goes up ~100feet then splits into three branches, two branches would reach the lower ceiling, the other branch splits again into 3 more branches, the 3 secondary branches reached to the top ceiling. The ceiling itself is made of sunflower like decorations. I felt like I was in some exotic forest.
On the right, there is a cross under a golden umbrella, with Jesus on it. There is a skylight above the altar - I believe that it symbolizes the connection to God. There is an area of pews in front of the cross. On the left is the main hall with columns on both sides. There are balconies on both sides - for choir, large enough for 1100 people up there.
When I looked up, I saw sunlight from windows large and small; some windows have colored patterns on them, and what is unique is that the windows themselves are formed via patterns of flower petals.
I would say that the most unique feature of Sagrada Familia, as the national geographic article said, is its nature inspired architect - externally and interally. It is a must see if one visits Barcelona.