Lately I have seen several threads that people have "tagged" as intended. Why is tagged in quotes? Because instead of putting keywords into the tags box, users are describing the question they're asking in the thread, and thus creating tags that look this way:
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So, here's my suggestion: Make it so that when the user types a new tag, after hitting spacebar triggers an event that turns the last word into a 'tag' element/box/whatever. That way, there's a more clear idea that what you're inputting into the textbox are tags, and is not meant for a sentence or a description.