Brad, that's a loaded question, and everyone here is talking about the performances and production, but none of them have mentioned the obvious: the technical side of it.

The acoustics of the Snakepit, combined with the stacks of Pultec EQs, compressors, and EMT plates, among other things, were the chief reason Motown had such a unique sound. Even the custom-built consoles, and the Altec monitor they used had an impact on that classic sound. That's why no one else got those sounds.

Stax recorded their classic hits in an old movie theatre, usually cut live, and in mono, until 1966. Stax didn't do too much signal processing. They went for that raw, organic feeling. They captured the moment rather than sculpt the sound like most others did.