Im not sure if there is an Updated set of doc's for public consumption on the specifics of moderation. Keep in mind each gallery has specifics for moderation, that fit with the type of skin being moderated.
Wallpaper for example is moderated including but not limited to asthetics,utility as a desktop wall,copyrights....
Where such as a widget, needs clean coding,"no malicious stuff added",asthetics,if it actually works as intended,copyrights,etc...
Of all the criteria for moderation,Lack of proper permission for ports and broken skins are the two most prevalent issues most rejections have, they either use copyrighted images without proper permissions or dont work correctly, if at all.
Read here for more info..Tho these are the basics, moderation takes other factors in to consideration as they arise.