Unfollowing someone because they don't follow you is stupid. In fact, many of the most insightful tweets I see come from accounts that don't follow me, mostly people who are to some degree high profile (@wycats (Merb/now Rails guy), @defunkt (GitHub founder), etc.)
If someone's talkative, it's pretty obvious, and I'll either get annoyed by their tweet volume and unfollow them after a time of dense tweets, or they're not bothering me so it's fine.
Quiet and inactive are a horrible reasons to unfollow someone, since if they're quiet the cost of following them is basically 0.