By comparison, you have Babylon 5, where the only humans were the folks in charge. Which makes those differences important. Take Delen, where her choice to be half-human had an impact with everyone.
If only
one guy in the group is human, then
he is the freak.
He is the guy that stands out.
Ah, nice counterpoint. An all alien group, with one "token" human works in a manner similar to the all human one alien group.
Now, if a game were run in say, Planescape, I'd expect the characters to be a mix of aasimar, teiflings, modrons or even stranger characters. A game set in the Shadowfell would likely have a group consisting of various undead. A group adventuring in a lawless triangle between a goblinoid kingdom, elvin kingdom and a dwarven fortress-nation would probably have a pretty wild composition.
But if the campaign were something like Greyhawk set in the great city itself, or in the FR Dales, I'd expect a party composition that was, well, rather conservative. I suspect most games fall into this sort of human-dominated take. Certainly 1E through 3E encouraged this, and 4E does to a bit.
However, I think if you look closely at B5, the only real "wild" main character I can recall is J'car. Londo* (my favorite) and D'len*, while clearly not human might almost be considered dwarf & half-elf** if they were fantasy races; their appearance was fairly human and their alien traits overall subtle. The remaining primary characters of the series - Garibaldi, Franklin, Ivanova, Sheridan & Sinclair, were humans surrounded by alien cultures - though those cultures tended to be "off-stage" pieces.
Star Trek does this as well. There is a huge variety of races in the Star Trek universes, but among major characters, there tends to be only one or two really unusual characters in a given serial. Spock in TOS, Warf & Data in NextGen, Odo & Quark in DS9 (Dax could arguably be an "elf"), T'Pol and Phlox in Enterprise. (Sorry, don't know Voyager well enough to comment there, but I think you get the gist).
* In this case, I am counting Londo's "moon-faced assassin of joy" assistant, D'len's assistant and J'car's assistant as minor/follower characters instead of main characters.
** I guess the entire Minbari race would be "elves"?