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I saw this article recently. I also know that human solid (not sure about liquid) waste is collected in buckets on river trips down the Grand Canyon.
What are some basic guidelines for determining when it is not okay to leave feces behind as they may pose a sanitary hazard and disturb the ecosystem equilibrium?
The basic guideline is do not leave your feces anywhere that it can be discovered or uncovered in the future.
As far as upsetting the ecosystem equilibrium, good luck with that, there are much bigger things than you in the woods are that are indiscriminately defecating on the ground and in watercourses. It's less of a sanitary hazard to the environment than it is to other human beings.
When disposing of your human waste in nature it's important to consider how frequently the area you're in is traveled, and whether or not your waste will break down where you leave it, or affect near by water sources.
In areas where lots of people visit regularly, it's unethical to leave your gastointestinally processed meals on the side of the trail or anywhere someone might step in it, see it, or smell it, or worse–ingest it by drinking from a violated water source. Specific rules may differ region to region, but always be considerate when toileting in the woods, if you have no other option, bury your business a good 60m away from any watercourses, trails, or gathering areas.
Mountaineers spent a lot of time in harsh climates (alpine, desert, arctic) where your waste is more likely to be preserved than it is to break down. In these areas, your petrified protein-rich poo could linger for years, and it's necessary to carry it out (or find a deep crevasse to drop it in–in places where it's still acceptable to do that). Visiting these areas requires planning on how to dispose of your waste prior to your excursion. Rock climbers will pack a wag bag, biffybag, or poop tube, appropriately sized for the length of their climb, and also plot out waste disposal sites where they can "lighten their load" (dumpster or outhouse at camp site they plan on passing by).
The biggest concern with human feces is exactly what that article you linked to was pointing out, that having too many people visiting an area, and not regulating their bowel placements, will eventually leave a place unfit for humans to visit. For the sake of preserving the beauty of nature so that many more people may enjoy it just as much as you have, leave it as natural as you found it, and leave no trace.
The only time I'm aware when you shouldn't leave feces behind is near bodies of water (within 200 yards of a body of water), which is why the feces are packed out of the Grand Canyon. (This is taking things too far in my personal opinion.)
