Well I’m not part of Twitter-TheCompany, so this idea is not production-ready … but I’ll try to give my solutions to these ;-)

1. No, you don’t get a message. If you visit his page you can just look it up or the Twitterclients implement their own customizable understanding of what to do when something in the Tags changes. I bet if this would become available, you could get E-Mails about such changes from external services.

2. Yes / No. Yes if your client doesn’t use the new API-element. No if you use the Twitter-web interface, where you just select from checkboxes which topic the tweet belongs to. And no if your client uses the new API-parameter for describing the topic.

3. @replies in Twitter always override unless you really block a user. But I guess it wouldn’t make much sense to block a topic, it would be unintuitive and contraproductive as blocking effectively means that its a spammer you block, so that option shouldn’t exist. So you’d get a Tweet from a topic that you don’t follow as you always get tweets with @replies, even when you’re not following a user.

4. As under 3., yes.

5. Its only an option. You’re free to follow anyone as you did before. BUT IF you realize, that someone talks about very interesting stuff about a certain topic that interests you – while talking a whole bunch of crap in between – it gives you the option to filter. If you’re interested in the crap too, you can just follow that person.

Ok, so if I just listen to 4 of your predefined tags what happens if you create a new one? Do I get those or not? Do I get a message every time somebody adds/deletes/renames a tag?

Do those “I tag my own message” things count towards the 140 character limit?

Can I also “Listen to everything from X except for tagY? If so, why not just use normal hashtags and allow people filtering their whole content based on it?

Do “@replies” override those tags (as in if I block everything from you with tagY can you send me “@teetante look at #taxY!” and I get it)?

If I block all things of you with “tagY” but listen to everything with “tagZ” will I get the message “#tagY #tagZ” ?

And finally (though you might disagree on this one): Isn’t part of the value microblogs bring the fact that you get some things that are not directly in your interests but that are from people you generally find interesting? It’s kinda how last.fm works: It suggests music to you that people listened to that have something in common with you, but the suggested music might be something you would have never dreamed about listening to yourself. That is part of the value.

Just a few thoughts.

@tante: Not really. I want to be able to tag my own tweets with a topic. Not using #topic, but one of the topics that I can set on twitter. When you’d follow me, you could click the follow button or the small “only topics” button. That would open a small form with all my topics and checkboxes for it with a button like “only follow these topics”. Therefore I could tag my own tweets with a topic so it only goes out to people who want to have it – or who want my entire feed. Untagged topics would fall into some “no topic”-category, so you can also get the usual blahblah while leaving out the unnerving Java-stuff that you don’t want to hear anything about.

After all, Twitter’s business is broadcasting A LOT OF information. Using a good way to tag your post besides using #topic would really help like everyone who uses it more than once a day. It also wouldn’t be too unobtrusive and would give at least these additional advantages:

1. you can set your topics as you can now set your bio. Everyone can see it and better understand what you like to post about. This is more straightforward that reading the bio.

2. You could better filter what you really want to listen to. Better filtering = more fun with microblogging.

3. You could get the feed of the topics of one person without having to search through his whole tweetwaste. If you see like “Whoa very interesting python-stuff he tweets about every 20 tweets” you could browse through his python-topic.

4. Nobody had to use it. Everyone could use Twitter the way it works now. Even the API-usage wouldn’t change if you only want the old function. Say: It degrades gracefully.

So you basically want to tag the people you follow and you want to be able to send a message to all people tagged in a certain way?
I guess the problem would be simplicity: People use twitter cause it’s simple. If you give people too many options that defeats the purpose and most people don’t have the problem you are talking about.
Just say you follow 200 people. Do you really want to spend all the time to tag them in order to sort their posts? And whenever you add a new tag you have to reconsider all the people you follow again to see if they fit into it.
So I think for twitter’s usecase this idea isn’t feasible. Identi.ca does it the other way around: you can add yourself to a “group” (say “Java”) and you can send groupposts (with “!java”) to reach all people that tagged themselves with a certain word (cause groups are kinda like you tagging yourself). It’s not exactly what you suggested but I think it’s simple enough for it to work: You just pick a few groups that you are interested in and get filtered content on those topics.