It's not that I'm that careful about privacy things in general. I don't care much that my grocery store card is tied to my real name so anyone might find out how often I buy razor blades or what flavors of ice cream I prefer. But the thing about these various offerings from Google (including the thing where someone can follow you on Google Reader) is that I can never figure out exactly what information is and isn't being shared.
You see, I have various email correspondents. Very few are private, but those few do exist. And I subscribe to a variety of RSS feeds in Google Reader, and I most definitely do not want them shared with everyone I email from my Gmail account (e.g., my coworkers, directors of graduate studies for the schools I've applied to, ...). I can't tell if Buzz does that or not.
What I like about Facebook is that it's a self-contained universe. Don't want something about yourself on Facebook? Just don't do that thing on Facebook. If you're secretly obsessed with manga, you don't have to share that with your Uncle Herbert or your best friend from 5th grade as long as you don't join some kind of manga-lovers group. You can craft your (more or less public) image however you choose. Since nothing manga-related is in any part of your profile, you don't have to know too much about the various privacy setting options in order to keep clear of it.
The various things I do with Gmail and Google Reader, on the other hand, are closer to my real interests, and thus I would rather not share them willy-nilly in a way I don't completely understand. In short, there is something to be said for not integrating services.