I'm Thankful for... Emperor Justinian I
The law applies equally to everyone in the Empire, not just the common people.
Studying the law helps man to get a greater understanding of the law - jurisprudence.
Previous judgement should serve as guides for current cases - precedence.
A man is presumed innocent until proven guilty.
Now, Emperor Justinian I also build a few monuments, won back parts of Africa, Italy, and Spain from invading forces, consolidated the empire, and married a woman with an unsavory background (showing a distinct lack of concern where social norms stood on such matters). But the concept that a person is innocent until proven guilty, that outweighs everything else in my opinion.
Human nature is to assume someone is guilty if that person is accused of a crime, unless there is overwhelming proof of innocence. To overcome this internal prejudice, we have to consciously assume someone is innocent, and have that notion pressed upon us in times of a jury selection or of a trial. But without someone in power who enforces this assumption, without someone like Emperor Justinian I to write down this idea, codify it and enforce it, I do not believe we would be the same world. We would live in a world that believe it's better for an innocent man to go to prison then for a guilty man to walk away free, which contradicts my belief that it is better for a guilty man to walk free then for an innocent man to go to prison.
As for the first three ideas, they all contribute to making the law fair for everyone and to remove the influence of money and riches over the judgement of the courts. I realize that money and power still influence the court system, even today. But think about how much worse this could be without Emperor Justinian I.