It's location on the river, opening up the city to the river. It's lines, very few of which are straight, flat, boxed, or perpendicular, are so languid, curving, and inviting. It's composite materials, titanium which gives it a sheen very similar to a fish's scales. It's unorthodox shapes, which are not even describable, are so interesting that they invite reflection.
As much as I am a fan of his willingness to try different ideas, forms, and materials, what impressed me more than anything were the size of the interior viewing galaries. They are huge. You never felt crowded. You had plenty of space to contemplate an object, sculpture, painting, collage, or picture. So much better on this score than the Guggenheim in New York.
The other really cool thing about this museum is the "sculpture" in front of the building. It is a Jeff Koons' 42 foot tall "Puppy" (a west highland terrier) which has 60,000 plants and flowers blossom through the steel mesh. It is just pure fun!