The ability to add/extend properties/datatypes to users such as a media picker (Thumbnail image or image), node picker, timezone, or what ever we need to add to the user like we are able to add to the Doc Types.
Of course keeping the core ones Umbraco uses uneditable for obvious reasons, but just the ability to add our own.
This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/56243-custom-user-properties