I made this setting because I hate the "average humans" trope. I never used it for anything, though.
Most humans thought that, when we entered galactic civilization, we would be "average". Not average as in "an ordinary species", but average as in "without any significant strengths or flaws, as opposed to all those damn specialists".
Of course, that turned out not to be the case. In 2160, when humanity finally encountered galactic civilization, we quickly earned the name "the outraged".
Our well-connected population had a finely honed sense of outrage: when something bad happened, sparks of outrage would flare up, and humans would unite to deal with it. If more humans were required, the fire would spread. This sounds like a slow process, but it really isn't: humankind has plenty of experience with becoming outraged, and has perfected the practice.
The first demonstration of this to aliens was when it became clear that there were violently xenophobic human groups. Humans became outraged when these groups began to operate, smashing them down socially and, when necessary, physically. This outraged the sects that were distant cousins of the xenophobes, which acted against the smashers, and in turn that outraged a significant portion of the species, and in the end there was a short but very bitter little civil war. It started two weeks after meeting the aliens, and lasted four days.
As humans and aliens began altering their biologies to cohabitate, it was quickly discovered that human outrage was not always pointed inward. The humans were the first on the scene to help the J'nal rebuild after their home planet was hit by a meteor, and they were the first to send military ships to stop the Pogulpogul from terraforming a garden world to suit their own needs.
It was assumed humans would join the galactic police and relief groups, but this didn't turn out to be the case. Our finely honed practice of outrage is more effective than any organization at directing our efforts, and the fetters of those organizations sat poorly on our shoulders. Naturally, "vigilante" and "human" became synonymous for some time after that, but as with all things, an equilibrium was eventually found.
As time passed and we integrated with the galactic civilization over a few centi-14C, their opinion of us became more nuanced. We're now making a name for ourselves as people try to learn our language, with its conditional meta-time dimensional probability conjugations of verbs.