I think if you get all the important and most requested features in there, the YouTube Editor could be a killer app.
Must Have Features: Edit long clips (seems like the 15min restriction has been applied to the video editor, but some of my uploaded clips are over 50min) Bulk Import video clips directly into the editor from PC (and store them in the "My Videos Bin" as 'private')
Import Audio (from URL/PC)
other nice features I would like to see: Import Photos (from URL/Picasa/Flickr/Facebook account or from PC) Export awesome clips like Animoto.com Capture long clips directly from miniDV camcorder in the "My Videos Bin".
My purposes for using the YouTube editor: Upload long raw videos (>1hr) shot on miniDV camcorder or HDD camcorder, then cut them up into relevant clips for viewing on YouTube. Upload short videos (<1min) shot on a Digital Camera and edit them into one clip with background music.
I probably wouldn't include any effects, text, transitions (other than cross-fade), etc unless they were up to Animoto.com/iMovie standards.
Looking forward to seeing the 15min restriction lifted primarily for people who can already upload longer clips.