This is the kind of question I wouldn't mind seeing more of; thoughtful & thought-provoking.
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To "credit" is to "publicly acknowledge someone as a participant in the production of [something]". The first part seems pretty straight-forward; "public acknowledgement... of someone". Most of the time in Wc3 modding, since the product is a map, we put something in the map itself; often in the Quest Log, but sometimes through text elsewhere.
But the last half; what defines a 'participant'? Assuming you are working alone, one might assume that the Creator, being the 'principle participant', would have a primary place in such Credits... But for whatever reason, this doesn't generally happen; it's seen as selfish or self-aggrandizing (or rather, it TOTALLY happens; one's name gets the place of honor coming after the word "By:" in the very title of the work!)
Now again, working alone but needing things one cannot provide by oneself, it stands to reason that those that provide custom resources that end up in the final product (models being the most common/popular, but also textures, icons, backgrounds, UIs, tiles, loading screens, splash pages, titles, music/sounds, spells/systems, etc etc etc...) should be credited in some way. Again, relatively "common sense".
However, what about those whose work does not end up in the final product (for any reason)? They still put in the time & talent (ostensibly in a Request) to help you. Further consider those whose help was not so tangible; friends and allies on the site who helped/supported/publicized (there's an argument that anyone posting on the public thread you made about your project 'helped' you in that way), the people who own & run the site, Blizzard for making the game, your parents for making you, your Diety for making the Universe...
Obviously at some point it becomes absurd. : ) But still, where you draw that line is up to you, yet still affects others.
Finally, you've got the list of everyone you want to credit. The final question is 'how'? Seems straightforward... But let me tell you, there's nothing I like less than seeing those dinky "credits" lists where it's simply a list of names. That tells me pretty much 0% of what I want to know: "who made X/Y/Z?" If I see a cool model, I want to know who made it so I can thank them for it, etc.
So the baseline for 'how' is simply that; a list of names. However, something I am personally looking forward to doing in my finished projects is a more... complete form of "Crediting". I plan on Crediting each contributor by name for their specific contributions. Further, a second, separate list of Credits "by contribution"; i.e. a list of all the models & junk, with names next to them so people can find who helped make what.
//EDIT// - 4300 posts woot!