I think that Dom3 does seem like more of an expansion than a full game, to the untrained eye.
That is because usually (that is, in the more commercial side of the industry
) expansions are merely modpack with a few more units and a new campaign, while often a new game just has less units and content (but better graphics).
Dominions 3 however, adds lots of units (and thus seems like an expansion) without adding new graphics (just seeming less like a new game).
In reality (rather than how things seem), Dominions 3 does add a lot, a new aging system, UI improvements, new shields, a random map generator, redone castles (and an AI that uses them), Dormant/Imprisoned gods, gameplay tweaks (costs/supply), much improved mod support, and I guess some more stuff I can't think about right now
That sure seems enough to me.