The biggest difference I have seen when people or communities switch from being predomindantly vegan to freegan is the moral implications. Veganism has manifested itself, both within the anarchist subculture and outside of it, as a hardline moral decision - those that eat animal products are bad/evil, ignorant, not revolutionary enough, oppressive, etc. Therefore, those that don't consume such products are good, in-the-know, making the right decsions, and engaged in a struggle for all creatures. "Being Vegan" becomes more important than the actual decision to not eat meat/dairy/etc. Being vegan means being held up to a higher standard, and if you fall short, or choose to live otherwise may just get you ostracized or punished (read: pied).
In my experience Freeganism still has the bearer-of-truth attitude without the moralistic generalizations of those who aren't in-the-know. That probably has alot to do the the "ill-defined, unstructured"ness of it.
I like the idea of a way of eating that's definition is a bit more fluid, and left up the individual to whom it relates.