"Dragonborn of Bahamut" are part of the Races of the Dragon splat book.
The dragonborn children of Bahamut are a unique race in that they are
not born; they are reborn. Each one enters the world as a halfling, an
elf, a human, or a member of some other humanoid race with all that
race's propensities and traits. Bahamut beckons to his would-be
followers, and those few who might choose to serve him.
The dragonborn does not become a "kind" of dragon, but is gifted with dragon-like attributes by the Good-aligned dragon god, Bahamut. You may choose which gift to claim but they are not type-specific. They do take on a draconic appearance, though to a primarily cosmetic degree. The template is a racial replacement, not a class replacement, and so does contain special bonuses but their comparison to a "spell-slinging rage wizard" is largely irrelevant as the two are not mutually exclusive.
The following article is available via Wizards' website but you should consider purchasing the book for more complete information: http://archive.wizards.com/default.asp?x=dnd/iw/20060105b
In FR the dragons were created by the primordials. Does this mean many/most/all dragon gods are also primordials, as aspects/fragments of Io?
The Dragon Gods are presented as actual gods rather than primordials (DMG p. 10, PHB p. 296, & SCAG p. 113).
Can a primordial also be (or somehow become) a god?
I'm unaware of this occurring, but I wouldn't rule it out:
- Asgorath/Io is both, but (as creator of the universe) doesn't count.
- Tharizdun is close, but is not a Primordial.
- Kossuth is also close, but he's "not a true god but actually an
elemental primordial".
Do primordials need, desire or benefit from followers?
Not normally, but Kossuth is an example of a primordial that benefits from extensive worship.
Do surviving primordials have significant religions associated with them, either in Abeir or in Toril?
On Toril the five Elemental Lords have followings, while The Seven Lost Gods were once worshiped. I believe Abeir would have more significant Primodial worship, but that setting was never detailed.
Do primordials grant spells in the same way as gods?
Not normally, but again: Kossuth is an example of a primordial that both grants spells and has extensive followings.
Can they be warlock patrons?
I don't see anything preventing this. A few homebrewed examples of this can be found here and here.
Is there any obvious difference in the portfolios of primordials versus gods? I would have assumed that since they predate mortals they tend to represent more fundamental forces (like the elements) rather than human concepts like law, love, luck, agriculture.
That might be one way to view it, but 4e's creation myth (Worlds and Monsters p.56, cited below) has the Gods being created at the same time as the Primordials. Another way to view it might be that the Primordials are composed of elemental "physical-matter" while the Gods (and Astral Sea) are more composed of thought (mental-matter).
In short, what is the difference between a god and a primordial? Are they fundamentally the same (just different lineages) or are there fundamental differences?
They are consistently presented as having fundamental differences. From 4e's Worlds and Monsters (p.56): "The gods, beings of divine power, appeared in the Astral Sea, while in the Elemental Chaos arose the primordials, incarnations of tremendous elemental might"... "composed partially of creation-stuff".
I also found this fantastically detailed post on Candlekeep...
That still seems like as good a guide as any.
Best Answer
Sounds like she's taking a page from Matt Mercer's Tal'Dorei campaign setting, where dragonborn with stumps for tails are second-class citizens treated as just a hair better than slaves and looked down on by the dragonborn with tails.
According to the Basic Rules/PHB, dragonborn in general don't have tails at all: