Do you really want to create a new tag?
In some of the SE sites that I frequent, I am not familiar with the existing tags. Therefore, I tag my questions with whatever category-name I can think of. Sometimes I invent a new category-name, or tag, that does not exist. I really don't ever want to create new tags as it is usually inappropriate. Therefore I suggest some sort of warning for when a tag is new and will be created. Even just highlighting it in red or yellow when the CSS kicks in would help. It seems to be accepted that under most circumstances creating new tags is a Bad Thing.
Here is an example question in which I had recently added new tags which I later removed.