I appreciate all of the comments; still unsure where to more persistently host the conversation, but for now I posted this still-arbitrary list on Commons.  (adding other items mentioned in this thread)      SJ

File formats: Support high-demand formats – e.g. CSV, CML, + hundreds of other open tickets 
Uploads:  Improve bulk, large, and video uploads.
   + Bulk conversion for video uploads (videoconverter / video2commons are broken)
   + Upload Wizard upgrades (timeouts, batch renaming, batch imports)
- Downloads:  Fix multi-download (Imker is broken) 
   + Make public dumps (stale since 2013)
- Video playback:  Debug + roll out the videojs player
Search:  Bring CQS back up.  Implement a noauth option for tools
General: Move to a blazegraph alternative (for wqcs) 
   + Images: Update thumbor and librsvg  ||  redesign the image table  
- Curation: Simpler content assessment workflow, like en:wp's (QI/VP doesn't scale)