Once it's clearly defined, it brings up the problem of "Well, what about so-and-so," which leads to "Well, if so-and-so can lift it, why not that person as well," and it keeps snowballing and snowballing until a whole slew of people are worthy, at which point the impressive nature of the feat becomes diminished.
But bluefall has already pointed out a way this could work.
If Mjolnir considers the virtues of a "worthy" possessor to include one who obeys his master and has the strength of heart to slay great foes, you get soldier Cap, warrior heir to Asgard's throne Thor, warrior guardian of his people Bill, warrior champion of Athena Diana, but not freelance technical pacifist Clark. Or maybe Mjolnir wants avatars who stand for something bigger - two gods who incarnate storms and truth, a guy who's the living embodiment of an entire culture, and the transfigured repository of an entire species' hopes would do it, but a guy whose greatest strength is that beneath his powers he's a perfectly normal average human wouldn't cut it. There are lots of totally logical ways to draw the line that exclude Superman but not other people who've held the thing under less exceptional circumstances
In both the whole Marvel Universe and the whole DC Universe, there are very, very, very few great and awesome and inspirational heroes who also possess those traits. The "has the strength of heart to slay great foes" alone could easily exclude all the great and awesome and inspirational heroes in MU and DCU but Thor, Cap and Diana. It can't snowball if there is no snow.