I would further add that Roman policy, in terms of domestic and foreign laws and attitudes, was absorbed and continued by the successor states with remarkably few changes. Theodoric the Great, ruler of the Ostrogothic kingdom in Italy, was technically the eastern emperor's viceroy for the western empire, and as such officially promulgated all the laws passed in Constantinople. This doesn't mean that they were given precedence, in practice, or that "viceroy" was anything more than a nicety, but it is a potent symbol.
The law codes of the successor states are also interesting, because they came fairly early, usually in the 5th century. Their purpose was to, mostly, affirm the rights of the non-barbarian population and also to put in writing the current practices among the barbarians themselves. So, for instance, the kings weren't claiming any right to adjudicate disputes between rivals rather than letting them feud, but they were stating the sanctioned times, places, and limits of feuds, as the current culture had determined them.
Later, in the early middle ages proper and then in the high middle ages, kings warmed to their legislative roles (for a variety of reasons), and took a more active stance on actually reforming their legal codes, usually in imitation of the Justinianic Code (the notable 12th century exception being Henry II of England).
driftwood