I am only a Q-list Roman historian, but I would argue that the biggest impact to come from the reign of Septimius Severus can be found in what he allegedly said to his two sons, Caracalla and Geta, on his deathbed:

""Be harmonious, enrich the soldiers, and scorn all other men."

The notion of placing the army above all of the other groups that an emperor would need to keep happy (the people, the Senate, etc.) really got of the ground with the reign of Severus. This trend was carried on by Caracalla, who gave the soldiers massive wage increases. The legions, of course, became used to their new income rates and discontent grew under Alexander Severus when he cut their wages, a big part that led to his eventual death but definitely not the only.

There are a few "popular" points of the reign of Septimus Severus that folks like to point out. First, that he was a "black" "African", so people like to point to him as an example of the "white-washing" of history. And those things might be true, but in terms of contemporary impact that would have been LESS than a footnote.

To begin, he was not likely what someone would call a "black" African anyway. He was descended from wealthy Romans in the matrilineal line and from Punic and Libyan nobility on his father's. The mixture probably rendered his racial appearance more like to that of an Arab. And more than expressing the "diversity" of the Roman empire, his candidacy for the purple represents the successful imperialism of the Romans.

Gibbon devotes (and I love to point my finger at Gibbon incessantly) a good portion of his "Decline and Fall" to the reign of Severus, but has, in the main, positive things to say about him. The real effects of his reign are as follows:

1. Restoration of dignity to the throne of the world. Severus' early service was under the reign of Marcus Aurelius (my personal favorite, and commonly considered the last of the 'Four Good Emperors'), and from his earliest days we was always considered one of that Emperor's "men". After the deposition of Commodus (Aurelius's son), a senator called Pertinax was acclaimed Emperor, but swiftly killed by the Praetorian Guard, who then literally auctioned off the throne to the highest bidder. Enter Severus.

Didus Julianus, a wealthy senator, won the auction, but when the news of Pertinax's deposition reached Severus at his post in Africa, his soldiers declared him Emperor, and set off to the capital where Severus had the murderers of Pertinax "put to justice" (decide for yourself what that means).

He then had to deal with all the other claimants to the throne who'd set out to do essentially the same thing. Most formidable of these was Pescennius Niger (commanding the Syrian legions) and Clotius Albinus (stationed at Britannia), whom he defeated in successive wars to claim un-rivaled mastery of the Roman world. Unlike most of his successors and most of his predecessors, he didn't anger anyone enough to put him to death. As a result, he

2. Founded the Severan dynasty. And boy was it a divisive one. His wars against conspirators and the Parthian Empire forced him to drastically debase the silver currency, which started an economic cascade that would eventually contribute to the famous Crisis of the Third Century.

Hope this is reasonable, and that it helps.