Just thinking out loud...

Code reviews... might be implementable in terms of tags. A tag directly on a commit (or wiki page, for that matter) would be treated as a comment. A tag on another comment essentially creates a comment thread.

The question, really, is how to store such constructs. Some random ideas:

A new Artifact Card, e.g. V-card (R-card is taken). Might look like:
V julianTime username OPINION uuid-being-commented-on comment

where OPINION is one of '-', '0', or '+', for positive, neutral, negative.

But i really don't want to add new cards - they are a huge backwards compatibility problem.

So we are left with a couple options:

Encode all info in the tag:

comment-username-timestamp-target_uuid=comment text

but that'd be really, really ugly.

So we store them as blobs with a new Control Artifact form. We can (at least in principal) introduce a new card type if it only appears in new artifact types :).

Hypothetical Comment artifact:

<nowiki><pre>
D timestamp
C comment-text
U user
V uuid-being-tagged ?[-0+]? (default opinion==0)
Z manifest-hash
</pre></nowiki>

(V comes from reView.)

Then we add a tag just like a Ticket's:

comment-UUID_OF_ARTIFACT

Hmmm. "It just might work."