Warhammer 40k III - Hats for the Hat Throne

[Winterwind]

Erm, to clarify, I am the only one who won't be buying a boxed set, as I've already purchased a daemon prince, one each of thousand sons, berzerkers, and regular marines plus a rhino and obliterators. Everyone else is using a boxed set*. I am getting the impression that since my stuff is more expensive points-wise I may not even need all of it.

Nah, that was clear - I merely intended to give you an idea of what such an intro box contains, on the hope that it's similar for different races.

It was brought up that only the Tau starter set comes playable as-is... What do the others need? Specifically Tyranids?

Okay, let's see...
According to Games Workshop's homepage, that intro box contains:

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Contents: 1 Tyranid Warrior Brood (includes 3 Tyranid Warriors), 1 Tyranid Hormagaunt Brood (includes 8 Hormagaunts), 1 Tyranid Carnifex, 1 Tyranid Termagants Brood (includes 8 Termagants), 1 Tyranid Genestealer Brood (includes 8 Genestealers).

Hmmm... actually, that's a legal army. The Warrior Brood is a legit HQ choice, and with the termagaunts, hormagaunts and genestealers, that's even 3 Troops. And each brood is big enough to be legal, if barely.
I'm not sure how effective this would be, but it sure works.

EDIT: Groan... double-ninja'd. >_<

Additionally, the Assault on Black reach set, where is it in comparison? I'm considered the 'resident expert' and I know nowhere near enough about any of the other factions to qualify as such.

What do you mean? It contains slightly less Marines than a battleforce box for Space Marines would contain (and the Space Marine force is not legal, lacking a Troop choice), same for the Orks, but together they count a fair bit more than a single box.