From Flat to Fluid
The main improvement is the fluidity in the design, in my opinion a perfect adaptation of Microsoft Fluid Design to the icons of its own applications.
While the old logo is 100% flat design, the new one could be flat 2.0 if it were simply this logo, by the shadows and gradients addition:

But it's not just that, the new logos are designed in the space, allowing a more dynamic and adaptable use, very representative words of our current digital and Cross-platform world.
It's not a flat element as in the typical icons, but its design based on modules allows a wide range of interpretations favoring its adaptability such as:
- Different perspectives
- Multiple shadows types, by position and graphic representation
- 3D thicknesses that appear and disappear or change according to the position in the space
- Brightness, yes or no
- Elements dismemberment

Conceptually pursues the idea of objects built from independent pieces, such as a Lego, something that not only points to a proximity to the receiver but also invites to use these same pieces to create their own adaptation. By the way, this is clearly shown in the video presentation.

While the typical application icon is the one shown without any alternative other than to look for a new adapted graphic, these icons offer all the possibilities without hardly altering the design.

Personally I'm not a big fan of MS products, but I see in these icons an excellent work, at a formal and conceptual level. One step ahead in what refers to icon design.
:)
- Personally, I think all the "letter in a colored box" logos are merely lazy design. They work, but they're lazy.