One thing that always bothered me about esotalk, especially since it requires mouse clicks to select tags, was that when I wanted to start a discussion I wanted to start typing right away. The tag selection always got in the way and distracted me by making me think of where a discussion I haven't even written yet belongs.
As much as I like the current popup I would like to see tag selection integrated into the composer box itself, so you can pick the tags at your own pace, while you write. Alternatively as a quicker solution, the tag selection could be presented to the user AFTER you hit "Post Discussion".
If you're worried about people not picking any tags with the former suggestion, then both suggestions could be combined: The tag selection box is showed after you hit "Post Discussion" but only if you haven't selected any tags while writing the discussion.