Back on topic:
wizgriz I'd prefer to keep tag selection in a modal dialog. I think it's better to present a full list of tags, front and centre, to make users focus on their tag choice.
That said, I see how it could be annoying if you're forced to pick tags before you can start writing. How about if we open the composer straight away, and have a little "choose tags" button to the left of the title that opens the modal? And as you suggested, if the user hits "Post Discussion" without selecting any tags, we show the modal anyway.