I wanted to add something not so new, but not often discussed in the online dialogue regarding Mastering. Our job role as mastering engineers extend beyond the importance of dynamic control and tonal changes, but to the dialogue and rapport, we build with our clients through communications, the ability to quality control any unwanted elements that have slipped through onto the mix and also understanding and appreciating the musical nuances of different productions.

Quality control;
Intermittent artefacts; Clicks, Pops, Glitches
Constant Artefacts; Hiss, Humm, Room Resonances/Tones
Identifying Audio Signal Dropouts
Identifying Digital distortion/clipping
Identifying poor edits
Identifying compression pumping/ducking in the signal
Identifying tonal imbalances
Identifying dynamic imbalances
(I'll be here all day writing this list... )

Professional Mastering Engineers; The Unsung Heroes 🏆