Anything that would allow users to set their own colours or background images would be great.
On those skins that allow an admin to change the background, it would be great if users could also select their chosen image (even if just from a small selection, rather than any uploaded image)
I think anything that is a choice for the admin should be selectable by users.
The greater the ability for users to customise their experience of the forum the better. Most will not do it, but the option is there.