A few things came up:
What happens if someone you don't really care for sends you a friend request. Do you oblige and accept and then inwardly resent the fact that this person is now privy to your goings on? Do you accept but only allow them limited access to your profile, thereby making it clear you aren't fully sure/committing to the whole friend thing with them? Or do you go full-out and actually reject their request, making it really obvious that you are definitely not interested?
What if you accept a friend request and then remember some really shitty thing this person did to you - esp relevant with people you haven't heard from in ages, seen since school etc. Do you send them a message pointing out the remembered wrong and discuss it? Or simply remove them as a friend? What is the etiquette here?
Or, and this is one I encountered when I used my blog as a social diary (back when I actually had a social life), what happens if you go about happily posting stuff you are getting up to - either via photos, your status update, or through notes, and other friends get huffy because they happen not to have been invited/included? Surely a definite downside to making your life so visible to so many people.
OK - so those are some of the things that came up. Personally I am too knackered to have an opinion/answers to any of these questions today. Feel free to send in your two cents worth.