I'm not crazy about the way user identities are tracked and connected across different sites these days. This is becoming more and more prevalent.
Actually, that's not quite right. I like the concept, I love being able to log in once (e.g. using my Google ID) and have many sites remember my preferences, history, etc.
The problem is the execution of this concept. In many cases, it's done in very non-transparent ways. It has happened to me more than once now that I posted a comment on a blog or news site, and it just "magically" conjured up a seemingly random avatar and attached it to the post. Now, like many people, I have various avatars and identities. For example, I have one for professional sites related to my job and programming in general, and I have another for (video) gaming, which I use on gaming blogs and sites. It's frustrating and slightly embarrassing when my gaming avatar suddenly shows up on a professional site, without any kind of warning or verification.
Some sites handle identities quite well. On those that do so, my user id and avatar show up on each page so I can see and edit it and sometimes even shows up right next to text I am editing, before I post it. Unfortunately, these sites are still not the norm, to the point that I am sometimes loath to post because I don't know what will happen if I do.