What we are talking about here is a combination of small beats in different parts put close together, which prevents the listener from hearing the “groove” of each of the parts. As a result, the track delivered to music mixing services becomes a mishmash of successive 16th or 32nd notes. This applies not only to drum parts or percussion, but also to rhythm guitars, keyboards and different effects. Rhythmical organization of sounds and pauses will make you feel the movement “groove”, which is necessary to receive a feedback from the listener in the form of dancing to the tune.
Thirdly, notice the elements that conceal each other in the frequency domain. They present a big problem for mixing. Such thing often happens between guitars and a digital piano, and between a piano left hand and the bass. It’s less common for pads, backing vocal, wind or bowed instruments. Harmonic instruments played in the same tessitura will inherently conceal each other. If this problem wasn’t solved during recording, then the music mixing and mastering studio will have to tackle it. Timbral transparency is a chance for listeners to enjoy the beauty of every part. Otherwise, he’ll have to trudge through a concrete wall of sounds falling on his head from the speakers.
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And the last issue is simply the mess that hasn’t been cleaned during editing which turnes mixing engineer's work to hell.