Since the MP/STG 44 was the concept upon which future assault rifles were based, its characteristics are the basis for the definition of "assault rifle".
Select fire; both semi- and full-auto operation.
REDUCED-power cartridge (if it is "high caliber" [whatever the h*ll that means] or "high powered", it's not an assault rifle).
In more recent times, most all military rifles have come to have black plastic furniture, with a separate pistol grip, and that appearance is thought by some to actually be the most important characteristic, though clearly, based on the definition, the M2 Carbine is an assault rifle even though it's not black, has no plastic, and is of coventional, that is, 19th Century, metal and wood configuration.
Today, any number of rifles that do not conform to the definition are termed "assault weapons" precisely because they are not assault rifles; the latter has a specific definition, while the former could literally mean any gun.