Currently to add tags and fields you have to visit each media item one-by-one after the media has been added to the gallery. This is generally too much of a faff for users, especially if they have added several media items. They just can't be bothered to go to each one afterwards to include the information. This particularly devalues tags - I find much of the media on my site isn't tagged and this is at least partly due to this.
I propose that tags and fields are added to the Add Media page so that, as media is added, tags and fields can be completed at this stage rather than having to wait until after the media is added (although I'd still have the current tags and fields tab remain so that any tags can be added/changed afterwards too). I think by adding this, it would enable greater use of tags and fields.