I believe that people with underscores in their usernames (ahem, like me) cannot be tagged. I'll think about giving myself a new username -- this was a problem in Jive 7 as well, so I don't expect Jive to address it any time soon.
Just typing mike (or Mike or mark or Mark) and there are too many to even show in the "possibles" result list.
But to add to this discussion: "tagging" is not always working with reply-by-email. So even if we had a non-underscore name, it would not appear as it does when entered on the forum.
@someone - and I know this is not valid
There are a couple other quirks (such as the square brackets getting munged with reply-by-email), so I tend to login a lot more.