The Louvre is spectacular based on it's sheer size and ornate facade.
Inside during open hours it's a bit of a nightmare. Not only is it insanely crowded as the world plows their way to see the Mona Lisa, but walking those endless corridors and giving each piece it's due time is a surefire way to drop dead from acute museumitis. It's gloriously overwhelming.
At night, with the crowds gone, it becomes kind of a public gathering space. People rollerblading and skateboarding and drinking from tall cans of beer.
The I.M. Pei pyramids are unapologetic in their ultra-modernism in such a grand, stoic setting. Paris is such a perfectly preserved old city that any time you see steel and glass it's a bit shocking.
True fact: in 1939, with the occupation looming, almost every last bit of art and sculpture was packed up, put on convoys and shipped off to safehouses all over France to be hidden until the war was over. Amazingly, nothing went missing and almost nothing was damaged.
As part of the grand plan there is a straight line going from the courtyard of the Louvre, through the Tulleries, Place de la Concorde, Champs-Élysées, to the Arc de Triomphe. It's quite stunning to see everything lit up at night.