I'm not a particularly heavy Calendar user (I mostly attend meetings others have created), so I haven't experienced as many problems as others. But here are a few things that annoy me:

• I can't accept a calendar invite from the preview pane. The invite needs to be opened first. (This isn't much of a problem now that I've turned on auto-accept.)
• I can't click-drag in the calendar to create a meeting with a particular duration.
• I can't see who has accepted and who has declined a meeting (unless I'm the meeting's creator).
• I can't quickly view a person's busy/free time without having to go through the motions of creating a meeting and using the Availability tab.
• The "Today" button randomly stops working.
• Scrolling up in the preview pane for a calendar entry is "sluggish" - it takes 3 or 4 times as many scrolls with the scroll wheel to scroll up as it takes to scroll down.