Imagine if the reddit homepage filled up with posts only from categories you explicitly followed and you might get something similar. There's still the problem of hundreds of people trying to have a discussion in one place, though.
Facebook connect is for simplicity during signup, and so you can post shared things to your Facebook timeline.
That's exactly how I use Reddit, and I was under the impression that pretty much all of the typical Reddit crowd does the same, no?
Reddit wasn't attractive to me unless I spent some time upfront to subscribe to bunch of sub-reddits and now I see only the categories I want to see (other than the top entry which is sort of what Reddit pushes as an Ad-posting).
Discussion at Reddit is indeed a problem, particularly for popular sub-reddit. I believe Branch is working on resolving some of the discussion issues apparent across the web.
Facebook connect for a new social service is counter intuitive. I will never sign up for a new service via FB or anything else.