What are unique properties or advantages of tagine?
There are no intrinsic advantages to using a traditional earthenware Tagine vs. a cast iron one or a dutch oven or a Römertopf or any other regular steel/cast iron or earthenware pot that people use to make stews.

Some/Most say the lid with its shape help the moisture drip down to the base, but so does my flat lid on my pot (or the small dimples on some cast iron lids).

There's a discussion on reddit and here about it.

iMO, if wanting to be traditional, use an unglazed earthenware Tagine, and use it often and be careful when using it; but if you cannot find one an use a enamel cast iron one, then there is no difference.