COO collecting can be a great way to collect, if you don't go over the top with all the made up definitions floating around today.
Every one feels the need to put a title in front of common coo variants, for eg: Palitoy, Meccano etc, followed by a big price, this is false as most coo variant were packed into various cardback, and same cardbacks had diiferent coo packed into them. Some COO variants can be a little more difficult to pick up, they will carry a bit more to the price, but 95% of the COO are quite common.
Variants to me are the different coo stamps on the back of the legs, each coo stamp has a pair to it, which usually has a very minor difference to the pair of stamps ( this falls into the category of mold family ) each mold family usually has details which can be attributed to that mold family.
Paint differences I prefer to put in the category of versions of a certain coo, not a variant in the sense of the word variant. A single COO can have paint differences in colour.
Certain capes, accessories, weapons, will usually go with a certain mold family.
It can be quite fun chasing variants
Then there is always the oddball stuff, unpainted, defects can be interesting too.
Variants is an area of the hobby that you either agree with or you don't, if you do, you quickly become obsessed chasing variants, if you don't, well you don't, that's fine.
I like what Pantzman said, "It can be as deep as you want to go"
Just look at Wolffs collection, close to 700 or 800 different figures. Set by COO stamps only.