Methods of tag entry for new vs. existing topics

I really like the process for adding tags on an existing topic. It's fast, accurate, and the instant-feedback it gives you (enter tag, hit return, it's added to the list right away, enter another tag, etc) makes me want to keep adding tags. I like "building" the list up from nothing. It's fun! :)

I don't like the process for adding tags on a new topic. It feels slower (because it lacks the "building the list" insta-feedback), and is more error-prone. I'd say a vast majority of my users miss the super important "comma separated" instructions, and wind up with a single tag with 10 words in it... That is, if they enter any tags at all.

What do you think about replacing the "new topic" tag-entry widget with the "existing topic" tag-entry widget? I think it will increase users' accuracy when entering tags, and it may also increase their likelihood of entering tags at all.

Aaaaand just to slide one more thing in here... how about autocomplete for the tag entry field? I've got lots of tags that all represent the same idea, but are themselves represented in different forms. For example, some topics are tagged "video server" others are "videoserver." Same idea, different representations.

I imagine this happens because users don't have a good way of seeing what tags already exist at the time of tag creation. I know there's a list of popular tags on the new topic tag-entry widget, but they're too far removed from the user's focus, and they're not context-aware. What if they want to enter a tag that isn't "popular" yet? An autocomplete field would bring the list of possible tags closer to their focus at the time of entry, and would also become more and more relevant as they type more characters.

I'd be curious to hear what others think.

-John