A lot of speculation has been said about this, I am going to try and figure it out -
First lets see who has held it before and look at common similarities. ( I have excluded a number of people who held it for reasons other than worthiness )
Wonder Women ( Alt verse but would be worthy in 616 also due to reasons Going to be listed )
|Thor||Warrior lifestyle - Willing to Kill - Think of others before self - Honorable - Very powerful - Sense of Nobility - Humility|
|Captain America||Warrior/soilder lifestyle - Willing to Kill - Think of others before self - Honorable - Not that strong - "Super soilder" - Humility ( pre-S/S serum )|
|Beat Ray Bill||Warrior lifestyle - Willing to Kill - Think of others before self - Honorable - Very powerful - Possible Nobility -Humility ( Asking Thor for help )|
|Wonder Women||Warrior lifestyle - Willing to Kill - Think of others before self - Honorable - Very powerful - Nobility - Humility/humbleness|
So a few clear traits show,
Being a warrior, Willing to kill, selflessness , Honor and possibly have some kind of status or nobility andHumility
The nobility trait may be said to not be true, but looking at all the people who have lifted it they have some kind of title, even Captain America is not just a solider but a "super" Soldier. He is also willing to kill, as he has done in wars.
A few people who are not worthy
Spider-man - Not willing to kill, Lifestyle, Not noble. Not really honorable.
Superman - Not willing to kill , Not really a warrior lifestyle.
Hulk - He does not have the warrior lifestyle, No honor, No nobility.
These are just some ideas, May or not be true, but they make sense.