ANNOYING: People who blab your ear off...
We all have at least one coworker, friend, or relative like this.
I don't think there's anything wrong with someone who's overly talkative -- that is, until he or she prevents the other from getting a word in edgewise.
I also take issue with people who talk over or interrupt others while they're speaking. Not only is this majorly rude, it's disrespectful.
In order for a conversation to flow smoothly, both parties should take turns listening and speaking. If one person is hogging all the time for himself, it makes the exchange seem too one-sided.
I quickly lose interest in continuing a conversation once I see the other person attempting to hijack it. As interesting as the person's stories may be, one who speaks only of himself and doesn't bother to take interest in the other person strikes me as self-centered and boring.
Don't feel bad for steering clear of blabbermouths -- they're everywhere!