There are persistent substances, like salt, that one would want to avoid.
The other, more specific rules some have for their compost are not necessarily based on whether or not the material will compost, but more about "icky" factor, and decomposition time, and are personal preferences. If it's organic, it will decompose. Organic is much more than "part of a plant." Plastic is not organic.
I put about any kind of discarded "food" in our compost, unless it's something our dog would find tasty, in which case I just give it to him.
The organisms/critters that perform the decomposition are where the "icky" gets involved, not the actual items in question. In the case of too much moisture, there's an odor from microscopic organisms & their anaerobic decomposition, the gasses they release.
In the case of meat scraps, there could also be maggots, which definitely release a foul odor. And most people are freaked out by the sight of them. A frozen piece of rotten meat would not have an odor, because the decomposition process has been suspended. Let it thaw out for a few hours, so the decomposition can resume, the odor will return. I'm not sure it's possible to compost flesh w/o odors, but it will definitely compost.
Some things just take a long time to decompose and aren't usually put in, like bones or larger pieces of wood or bark, corn cobs. This is also a personal preference about decomposition time, not whether or not it will decompose.
I've found it's much easier & less time-consuming to put OM in its' final destination the 1st time I move it & skip the whole pile thing. I now think of it akin to starting a cutting in water instead of soil, an unnecessary extra step. It was fascinating for about 15 yrs, and worked well in OH, but the novelty has gone here in AL. I know some folks don't mind or even enjoy it, but I don't want to spend time moving heavy loads of stuff, or stirring up a compost pile, and the ants eat most of everything when I make a pile here, leaving nothing to cart around to beds anyway. OM can be used on surface to augment mulch, slightly buried, or loosely piled between plants in small enough amounts to not produce anaerobic decomposition. Whatever works for the load of stuff I have available on any given day.
