[dmnd]

I really like the idea. A few things I noticed:

Tagging a conversation with a pronoun like "I" will probably means it sticks around forever.

Can you join multiple conversations at once? What happens if two conversations share a tag? Are they merged?

To stop conversations being tagged incorrectly, you could perhaps ask users that join via that tag if it was accurate or not.

It'd be awesome if you could automate tagging of conversations. I'm not sure how many people will be motivated to maintain metadata about their conversations (the only way to find that out is to get more users though).