Assuming that you are talking references fur fuzzy characters, I'd start with muzzle and paw shapes.

Yeah, basically the details that make that furry character as opposed to someone else. I have a surprising amount of adopts and other characters that could use some more art of them. Myself too, come to think of it! And well, no one's going to know what to draw for me unless I make a guide.

Try not to go too overboard. Include all important markings, maybe include an outfit so the artist has a clothes reference. A closeup of the face/hairstyle is usually good. As someone who takes commissions, I look for things like tail length, number of fingers toes, whether plantigrade/digitigrade, and general body proportions on a ref.

This is a good ref: https://www.furaffinity.net/view/7062398/
As is this: http://d.facdn.net/art/flammiekid/1....._reference.png

I really couldn't find any examples of a ref with TOO much info- generally that has more to do with the character design being too tedious than the ref being poorly done. Unseen things, like a list of likes and dislikes is usually superfluous and best left to the image description than the actual sheet. A description of the character's general demeanor can be a good thing but that's info usually best provided when contacting the artist about the commission.

I would follow this basic rule- start by displaying everything about your character that you would like to see on a pic to where you'd look at the pic and make a positive identification. The simpler you make it, the less likely people will forget details. A strong sense of observation is not a trait everyone has so the more essential and straightforward the ref is, the happier the people you commission will be.

That's good. :) Thanks! I was concerned about how much non-visual/semi-visual stuff should be put on the image, but I think I'm happy with leaving it in the description part anyway. Thanks for the feedback!