I understood "tagging" more in a sense of "tag, you're it". Like some bloggers do, "I got tagged by Person A to answer these questions", I'm pretty sure I've seen that before. Maybe I got it wrong, but I thought was like what you just did, @mentions :P
As for tagging with words (and maybe even more than just one taxonomy), of course, it's the best. Tag first, tag lots, and find uses for the tags later I say :D
When I viewed the slide presentation, I only noticed the @mention type of tagging. But I had had a hard time going through the slides, and since your starting comment seemed more in line with the folksonomic type of tagging, I was kind of hoping I had just missed that in the hurried presentation.
In this day and age, I haven't seen any reasons not to support the @mention type of tagging.
The folksonomic type of tagging can absolutely be overdone or restricted into absolute uselessness. But when done properly, they can be extremely useful.
As for tagging with words (and maybe even more than just one taxonomy), of course, it's the best. Tag first, tag lots, and find uses for the tags later I say :D