I think it would be useful if the
[tag:] tag-linking syntax worked in comments.
[meta-tag:...] now work in comments, with the same semantics as in posts. They are however not styled as tags, but as plain links (with the link text being the tag name); we don't want excessive styling in comments.
[tag:] syntax generates objects slightly taller than plain text and causes some modest layout artifacts. I suspect these would be pretty distracting in the comments, so I'm not very motivated by this idea.