There is also an emphasis on performance, with "good" as part of a demonstration of proficiency. It need not just be about behavior, and I think this performative aspect is as evident here today, with those recognizing the "good" in others as they speak about their work, as it is in online and virtual environments like World of Warcraft. Various and sundry forms of aptitude all relate to some understanding of "good."
As with most all words, though, "good" can be complicated in practice. This picture of Richard Nixon making Elvis Presley "federal agent-at-large" in the Bureau of Narcotics and Dangerous Drugs is the most requested photograph from the National Archives. Questions of "good" still surround both men as the word also holds implications of who is estimable and deserving of our respect. There is the question of ethics, i.e., was Nixon a "good" man, a good president? There is the question of proficiency, i.e., was Elvis a "good" man, a "good" performer? Again, there is the moral quality to "good" as well as the more obvious kairotic elements.
In response to Nixon and Elvis, I offer up this early example of the facepalm. It also functions as an additional understanding of "good" as expression and movement. Just as "seriously" denotes a sustained manner, "good" does, too. Behavior and performance are both forms of expression, of movement through spaces offline and online.
Again, the performative element, that "good" is something acted rather than acted upon, that behavior and performance are not mutually exclusive endeavors but often the same. And we aren't "good" alone. I'm not sure we can be, either because of the philosophical "if a tree falls in the forest and no one's around, does it make a sound?" sense or because of the element of public and social recognition. Others support us in our "good"ness, recognizing it as much as our seriousness.
To be "seriously good" is to be earnest and effective in expression.