Considering their function, pro-audio plugins have a very dominant visual component. The quality of the user interface can add definite value to the UX and win customers away from a competitor. Waves Audio Ltd. makes great quality audio plugins that lack visually. They hint at skeuomorphism but are visually scattered across the spectrum from photo-realistic to a basic gradient.
Circular knobs are also an extremely efficient UI mechanism, often found on audio equipment like guitar effect pedals and rack mounted units. In theory, one may fit three knobs with padding in the same area that a track from a slider would take if the width of the slider's track and the circumfrence of the knob are the same. (C = 2 π r)
In addition, the perceived gradual nature of audio (many of the numbers are perceived to ease in and out when listening) the visual of a circle provides a convenient vehicle.
The interface elements are also designed in a way to allow function in a touch input based environment. This may be more relevant in the future as more tablets are equipped with full operating system distributions and touch panels become more affordable for desktop and laptop workstations.
Any sliders have a text field companion where the user can input a desired number directly into the program. In addition to making the suite more accessible it is a value added feature for advanced users that will calculate various measurements relating to their sound like delay/reverb timings, compression settings or phasing based on their signal’s input metrics and various equations.
When stereo audio is panned (a process of splitting the signal more to the left or right bus by a set amount) it is often split along a curve as opposed to a linear one. For example, 30° is interpreted as half way, 50%, to a side as opposed to the linear interpretation of 45°.