They could work on a conversation view, and to not count the @mentions in the character limit.
But I guess if you didn't count @mentions in the limit you'd find people adding lots of people to a conversation in a spammy way. I'm not really sure how to fix that.
Perhaps there could be a meta-@mention that would include all previous recipients in a given conversation. But then how do you remove people from a conversation? Arggggh!
(Still, i've only thought about this for 2 minutes. I'm sure there's a way that makes sense if you spent more than 10 minutes working it through.)
From there you could also offer to exclude uninvited users from the conversation (maybe it is clearer to say, limit postings to the conversation view to people that have been @mentioned).
Simple: cap the number of @mentions in a tweet at 5, or at 120 characters total.
Same for #tags — limit those to 5 or a total number of characters.