Then and Now
I've seen a lot of people talking about how the "Earth looks dimmer" now, but actually, it has to do with how the photos were captures.
The photo on the left (the famous Blue Marble photo) on the full visible-light spectrum and was taken on film. It was taken from the Apollo 17 in 1972 as they moved towards the Moon.
The photo on the right (the new photo) is a composite of digital images taken in narrow bands of red, green, and blue. It's also taken from much farther away.
So, what will they do with this photo?
Well, they plan to create some data processing techniques that will allow them to remove the effect of light being scattered by air molecules (the blueish tint you're seeing) so that they can emphasize land features.
Then, the data that the EPIC begins collecting every day will be used to check the levels of ozone and aerosol in the atmosphere, while also looking at cloud height, vegetation properties, ultraviolet reflectivity and more! NASA hopes to use this data for a number of Earth science applications, including dust and volcanic ash maps of the entire planet.
What does the "whole sunlit side thing" mean?
This picture is being praised because it is the first in a long time that has the "full sunlit side." What does that even mean? Well, actually, it just means that most satellites are not far enough out to be able to capture this. But EPIC is at the L1 point (where the gravity of the Sun and Earth cancel each other out), all photos from here will be of the "entire sunlit side of the earth."
Why? Because EPIC collects data by taking 10 different images at different bands (all at the same moment) and then these are compiled. All the other "blue marble" photos we have gotten from NASA throughout the years have been mosaics stitched together--this is a composite of moments at a single point in time.