I think having all the fields available on new entries would scare a lot of new people away from submitting. I think that's one of the reasons it was originally setup to have just the necessary fields. So inexperienced submitters could feel like they had done something without being overwhelmed and then a more experienced submitter could come along later and fill in the rest.
With that said, I think it would be awesome if there was a check box at the top of the submission page for "show detailed submission page" so anyone could check that and all the fields would then be available to them. It would take an extra half-second, but we wouldn't have to worry about a massive page scaring anyone away.
halfway through your post i was going to suggest something to switch between a simple and advanced submission interface, but you beat me to it.