That hasn't bothered me too much. When you're looking at a discussion, you're mostly thinking about how to respond to that discussion. And the "Start a Discussion" button really needs to be accompanied by the list of tags, so users can think about which tags would be appropriate for the new discussion before actually clicking the button.
On the other hand, making "Start a Discussion" available everywhere might help cut down on off-topic replies to existing discussions. But design-wise, it would mean reducing the amount of space available for displaying the content of the discussion.
Unless ... here's an off-the-wall idea. Perhaps when in discussion view, the "Start a Discussion" button and the list of tags could be displayed in a slide-in panel? (By this I mean something like the panel that currently slides in from the left to display the "All Discussions" list.) Since the "Start a Discussion" button and tag list should slide in from the left (to match the layout of the discussion list view), the current discussion list slide-in panel could be moved to slide in from the right instead...
