3. Minimum mixin requirement on all transactions (with some special exceptions for existing "dust")
4. No new "dust" allowed.
Where can I find the definition of "dust"?
Mostly in the code.
The idea is anything not conforming to N * 10^M where N is 1-9 and M is an integer.
Since these values are limited in number within the range of plausible amounts (a total of 181 are currently possible but some of these are implausible) there will always be available outputs of the same size to mix with, in turn allowing mixing to be required.
Unfortunately there were a few edge cases not handled correctly and some new dust will be created in some cases, but the amount will be vastly reduced. This will need to be cleaned up in the next fork.
The other definition, of amounts that are simply "small" (such as 10^-12) we are allowing to be handled on an economic level for now. For example, miners are allowed to claim less than the full reward if they like, which means they can exclude the very small outputs. This makes their coinbase transaction smaller, which means it will verify and propagate faster (and of course leaves more room for fee-paying transactions). By default this is cutoff at 10^-5 but that is not forced, if miners disagree with that value they can change it.
I will add a link to this post in the OP. Please let me know if there is some Aeon content I should add there too
