My humble opinion about Letizia's gala appearance:
1. The make-up is soft-toned utterly refined
2. The shell-shape-with-pearls, low-height tiara is a masterpiece (who by??) and so are the matching earrings.
3. The white top is a "large fit" but, in disagreement with some esteemed discussants herein, I feel, it is right and compliments her due to her slim structure and the fabric's heaviness [does it have paillettes and rhinestones - couldn't tell exactly] and texture.
4. The skirt adds contemporary chic and mellows the severity of the black-white sharp contrast.
On the negative side:
Due to its size and the green-and-yellow/gold color-code, the decoration(?)/brooch(?) diminishes the drama and overall elegance. It should have been on the same scheme as the tiara and earrings.
A big "boo-boo":
The width of the red sash was extremely big and overwhelming [it should have been 30% narrower]. It made her look as if she never wore a sash before, much thinner than she really is, and diminished the overall effect.
Overall:
Outstanding appearance. It is sad though that, although it could easily have been perfect, it wasn't.
For the first time, in my eyes, she looked middle-aged. There is nothing wrong, of course, with being middle-aged but looking middle-aged before one's time is problematic.