tl;dr: Jellyfin 12.0 drops the leading "10." from version numbers and builds on the 10.11 database rewrite with major performance gains, particularly for playlists and collections (now stored per-row rather than as monolithic blobs). Notable additions include proper native support for books/comics (absorbing much of the Bookshelf plugin), alternate versions for TV episodes, per-library recommendation sources, and the Modern layout as default. Upgrading requires being on 10.10.7 or 10.11.x first, a full backup, a mandatory library rescan, and removal of third-party plugins; legacy `/emby/` and `/mediabrowser/` endpoints are also gone.