ZXSpectrum is right - design is not just a matter of opinion, there is an irreducible cognitive issue of the signal-to-noise ratio.

Every visual edge between differing colors and intensities is a stimulus to the eye and brain. Strong contrasts (e.g. black on white) provide stronger stimuli, and weaker contrasts (e.g. different saturations of the same color) provide quieter stimuli.

Wherever there is a contrasting edge that communicates no information, or a visual difference that is more pronounced than the minimum necessary, you have noise rather than signal.

Technical changes like increased color spaces (and some of the new UI possibilities in Leopard) open opportunities for the improvement of information design quality because they make it possible to lift the signal to noise ratio, typically by eliminating redundant edges and muting contrasts down to the minimum effective difference.

It's clear that Omni are not prioritizing visual quality in the way that some other companies are. You have only to look at the unneeded yellow background in the context sidebar, and the jarring contrast with adjacent purple text, to realise that not all the visual stimuli in Omnifocus actually communicate any information. Some simply distract.

For further food for thought, try "Envisioning Information" by Edward Tufte, or anything on Relevance theory.