For music:

Creating playlists:
Have an option by pressing the menu bar, "add songs to playlist", which allows you to select which (multiple) songs at a time to add to a previously or "now playing" playlist, similarly to how you can "share" multiple photos at a time by selecting lots of photos and then pressing the "share" button.

Editing playlists:
The ability to edit playlists more than is possible now, such as deleting a song from an existing playlist, or re-ordering the playlist.

Changing metadata/tags on the phone itself:
Allow right-click or mass (see above as with sharing multiple files) changing of tags. For example, if you've loaded a lot of music and haven't had the time to create tags, you can at least add things like "Album titles" or "genres". This would also be useful in File Manager for those (like me) who sort music into folders.

Album art:
My N82 allowed attaching album covers (or any picture) to a song when it was on the phone. These images weren't embedded into the music file - something I would personally prefer, as you can use a big high quality image without it being replicated in each song of an album, and inflating the size quite a lot.
Having read:
http://live.gnome.org/MediaArtStorag...ding_media_art
I would suggest this could be done on the phone by moving to a separate folder (if not already) and putting in a file called "cover.jpg" with the selected album art.

Quick search of songs/albums/artists by typing on the keyboard:
This doesn't seem to work too well at the moment. I have roughly 2500 songs on my N900. I'd hope that typing would bring up pre-tagged music/video files by album, artist and song name. It would be ideal if the typed in characters were searched for in any part of the string in the filename/song title/album/artist such as in a Symbian search of the complete phone. Sometimes, you only remember part of the name of a song, etc.

For videos:

Playlists:
These are useful for watching a bunch of TV shows sequentially (especially with TV out and lazing on the bed with the N900 way out of reach), or for watching short clips - anyone seen Dexter First Cuts? 4 episodes of 2-3 minutes each would be much easier with a playlist.

Tags:
As original poster has said. It doesn't necessarily have to be embedded in the video file. The N900 could have a data list of filename, location, tag. eg. TV, movie, documentary, music video etc. These tags would have to be added on the phone itself though, unless a file can be created externally or some sort of syncing to do this.

Optional User-defined Thumbnail:
You should be able to manually put in a thumbnail for the video if you are not happy with the default one. What happens if you have Basic Instinct on your N900, for example, and it randomly attaches the infamous crossing legs scene as the thumbnail?

Video grouping for Genres
I'm not sure about this, but I have videos on my N900 in separate folders, eg. movies, TV, cartoons, documentary, music videos, etc, which I use in File Manager to play the file. It's a lot easier that way than the visual method of finding what you want quickly. Perhaps the folder names could be used as Genres or Groups in the Video section, though I would think as a user-selectable on/off option.

Quick search of video clips by typing on the keyboard:
This could search for any part of the input string in the filename and filter by that. This would work for things like if you have TV shows on your phone. eg. All episodes of The Office have that in their filename, and all episodes of The Big Bang Theory would have that in their filename. This would help filter down to what you are looking for.

(edit:
added editing playlists)