The most blatant annoyance is that UI-wise, it consistently makes the wrong, unexpected choice in response to user interaction. And it behaves likes a blocking single threaded app that cannot download something and handle user interaction at the same time.

In addition, it's terribly buggy. Some things just go wrong from time to time with no apparent reason, and without non-cumbersome ways to recover from. There's lots of issues in the Apple support forums where the answer is just 'yeah, this sometimes happens, you'll have to delete this or that and re-add it'.