As you can see, letter on each icon have been reduced in size, with the remainder of the space used to show a highly stylized representation of the application. The colors are now a bit brighter, too. Oddly, there are already some inconsistencies in the designs, like OneDrive doesn't have a letter at all, it's just a cloud. Skype uses the old proportions, with a letter that's much bigger than any of the others.
As per Microsoft, new icons will begin rolling out across platforms in the coming months, starting with mobile and web.