This, definitely. Reddit's always had an "Other Discussions" button in posts, and I've actually found a lot of good small subs over time from clicking through and reading comments in other subreddits.
That's a great idea. What if it showed you a side by side of the most active thread along with the thread on your local instance (if it exists). Then include an "other discussions" button to see all instances with the post. As well as a "merge comments" button to view every comment in one page ranked by upvotes.
