BeFS's metadata support was much different from usual—it was a key/value map attached to each file. NTFS supports extended attributes and alternate data streams, which you could use to do the same thing, but none of the *nix filesystems have anything like that. What sort of metadata do you mean with “almost every filesystem ‘supports metadata’”?
JFS, XFS, ReiserFS, ext3/4, and btrfs all support extended attributes – that's become a bigger deal for things like ACLs – but along with varying size limits the share the same limitations as NTFS, HFS+, etc. of not indexing those values.
That means there's no way to do a query like "file type = audio/mp3" without either walking the filesystem or querying a separate index, which removes a lot of the benefits.
I'm not sure either – it definitely did not in the past but I haven't needed to look into it for years. Things like WinFS were going to solve that the right way but since that never shipped I assume they were still relying on a bolt-on indexing service.
