well the submission information is stored in a specific table. When the data is displayed for approval, its used in a template. Then, it is saved as an actual content in another table. in each place, the new field has to be added. Also the storage, loading and updating of the database has to be handled. To handle the fields of a table so generic that a non-php savvy admin can add fields of whatever type to the whole system is basically impossible, unless the whole system is setup as such from the beginning.
Not to mention the problems you get when an admin creates a field "x" and the actual function is extended also in a new version by a field of the same name "x". The data would be corrupted, the upgrade would break.
I agree with you that it would be sweet but I can assure you that you will not find many portals/weblogs/cms that can handle such a feature. :kickcan: