1. Applications, quizzes, and games: the biggest waste of time and electrons on the planet -- and that's saying something, because we humans have turned wasting time and electrons into an art form. And it doesn't even matter that I don't participate in them: they clog up the status update list. Facebook Lite (http://lite.facebook.com/) goes a long way toward fixing this. But some stuff still gets through, and I really wish we could also block "[Friend] posted a comment on [someone I've never heard of]'s status".
2. People making cryptic updates: instead of posting cryptic updates like "ouch!" and "I'm experiencing dueling grills" (real updates I've seen by the way, and that was the full extent of them), why not just say "please ask me about what is going on with me"? Except Facebook is the place where we are supposed to, well, just tell people what is going on with us. And if the update is an in-joke for a few of your friends, that's not very nice for the rest of your Friends, is it?
3. Posting too often. If you find yourself posting multiple times a day, think of the flow of the home page of all your Friends, and consider posting maybe a daily wrap-up update at the end of the day.
4. It's all so superficial, isn't it? People rarely talk bout what's really important in their life, because Facebook is just too public, especially when we have people as our Friends who aren't our friends in real life. I'm pretty picky about who I have as Friends, but even so I never felt comfortable posting real news.
5. Security/Privacy issues. Don't think for a second that things you post to the internet ever, ever go away, even if you delete them. And Facebook continues to struggle with where to draw the line between keeping everything private (what we want) and sharing everything (which makes them money, which is what they want). Also, one of the big reasons I don't do a lot of applications is that, when you start them, they say "in order to find out Which Type of Donut You Are, you have to share all of your information, and the information of all your Friends, with some unknown person, who will then do with it whatever is in their twisted little minds".
So anyway, if any of my friends are reading this, none of this probably applies to you, so don't take it personally. :-)
Learn how to delete accounts from many social networks, including Facebook.