I think what makes a good figure is one that is pleasant to work with. Dawn, Dusk and La Femme fall into this category.

I don't pull my hair out, cuss and scream, and generally groan when I get to the rigging part of making content for those figures. They are clean, well made models, without gimmicks and tricks that mean jumping through rigging hoops just to get a bend to work properly. They are enjoyable to rig for.

There are a couple other things I think make a figure successful in vendor support: a) not changing the creation environment so much that the vendor has to learn their craft all over again for THIS figure; and b) so many changes and differences that a long established work pattern has to be completely re-worked.

Those 2 items above are figure killers - vendors simply will not support the figure.