One of the main functions of a feed reader, as I use it, is a to-do list: it keeps track of what items I haven't read yet. This is an affordance that I find useful, but it's not really intended by the authors of feedreaders. This leads to some annoying deficiencies.
Google Reader only remembers items for a month. After that, it automatically deletes them, whether you've read them or not. This makes sense if you use the reader for browsing recent news, but not if you use it for remembering to read every post. For a to-do list, it is difficult to imagine a misfeature worse than automatically forgetting items.
I've lost some 30 unread items in the last week. Some were unread because they were uninteresting. Some were unread because they were so interesting that I was not done with them after one reading. Some were unread because I simply hadn't gotten around to them yet. I don't know how many of each there were, because Google Reader won't even show me what's expired. This invisibility makes the expiration misfeature far more annoying than it would otherwise be. I have lost part of my to-do list, and I don't even know what I lost.
I like Google Reader's portability - I can read my feeds from any machine. But I do almost all of my reading from one machine, so maybe I should find a reader that doesn't forget things. I can do that just fine by myself.