Well, it's not so much that his worthiness is objectively more superior than that he's more worthy by Odin's subjective standards.
But you bring up another reason I think it's best for worthiness to remain unclearly defined. 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.
As an aside, I've always felt Beta Ray Bill being from another galaxy is part of what I feel makes his debut story work. The idea that, in the whole wide universe, somewhere out there was another individual worthy to lift Mjolnir is nice imagination candy. But once you start making worthiness more common till the point where even in the United States alone, you can find a number of other worthy folks, the concept is diminished. To my eyes, anyway.