An often overlooked aspect of playing guitar that many beginners aren’t aware of is the importance of silencing the open low E string when moving from an open Em chord to an open D chord, or from E to D.
In most cases, you do not want to have that ringing open low E note “lingering” into the D chord because it will sound dissonant, muddy and sloppy, as the note does not musically “agree” with those that make up the D chord (D, F#, A), especially in that low register.