Equipment racks work best when they're strong, resistant to resonance and vibration, and above all, level. I don't mean 'slightly' level - I mean really level, using a spirit level. A good one, too. Years back (help me out here, hi-fi guys) the boys at Mana Acoustics furnished me with an especially accurate level that I still use to this day to set up anything I'm reviewing.
Why does being level make such a difference? Simply, because it allows the source component, typically a disc player, to devote more of its effort to reading the disc, and less to compensating for the errors created in reading said disc. The more level and stable your disc player, the better it'll sound. And yes, I'm aware that optical assemblies in CD players operate in nanometres, and yes, I doubt anyone could be so sensitive with their spirit levels that they could make that sort of adjustment - but that doesn't alter the fact that this particular bit of weird science actually works.
Want added evidence? The search for stability has driven many a CD manufacturer over the years, from Pioneer's stable-platter and TEAC's VRDS mechanisms through to the top-loading, puck-and-clamp designs popular at the moment in 'serious' hi-fi. Look at the latest generation of high-end CD players: built like tanks, one and all, and not just because they're chasing marketing credibility. They're all after the same thing - increased stability, and with it, accuracy.
The same benefits apply to any DVD or HD disc player, or indeed anything with moving parts, such as an HDD player. Of course, a turntable stands to benefit perhaps more than any other source.
Of course, other elements of stand design have an impact too: the proper control and dispersion of heat, in particular. But for all that - and especially for those of you looking to tweak your systems this weekend - do remember a session with a spirit level and a spanner (to tighten your stand's spikes properly) can be one of the cheapest upgrades you can make.