I'm pretty sure I read somewhere (perhaps in The Star Trek Writer's Guide
or maybe in The Making of Star Trek
) that one of Roddenberry's explicit edicts for TOS was that the main characters be written as basically modern-day men--complete with flaws, weaknesses, and even eccentricities--so that viewers could easily identify with them. That's certainly the way they were portrayed in most, if not all, of the episodes.
By the time TNG rolled around, something had obviously changed. I think, by that time, Roddenberry had so bought into (or, at least, decided to exploit) the fan bullshit that he was some kind of great visionary that he saw his core audience not as the general public but rather as an elite clique of enlightened "Trekkers" who liked to think of themselves as members of Roddenberry's "evolved humanity" and looked down their noses at "20th century man."