I know there have been some improvements to the Calendar section of the To-Do Bar in Outlook 2013 with recent updates, however there are three more things that need to be fixed in my opinion. I'll put these in order of most significant to least significant:
1. When I click on an appointment in the list, I no longer get a contextual tab in the ribbon allowing me to perform quick actions with regards to that appointment. This is a feature I FREQUENTLY used in Outlook 2010. I could click on the appointment in the To-Do Bar and there'd be a contextual ribbon tab where I could perform simple tasks such as: deleting the appointment, adjust the "Show As" and "Reminder" fields, etc. Now something that used to be a two-click action now requires me to double-click to open a new window, then click the action, and then click again to Save & Close. You would think with the in-line reply/forward feature that they'd want people to be able to perform more tasks without needing to open a new window.
2. The To-Do Bar's mini-calendar no longer indicates in bold the days that have appointments scheduled. This may seem minor at first, but when you're composing an email and attempting to see, at a quick glance, what days you might have completely open for potential all-day appointments, you now have to pop out the email (if it's a reply/forward), minimize it, go into your calendar and look for potential days, then restore your message to continue. It's actually a real hassle. I only list this as lower priority than the previous because I don't do this as often, though it is even more of an inconvenience when it does happen.
3. I can only view one month at a time in the mini-calendar, no matter how wide I stretch the To-Do Bar. Combined with the previous issue, I used to be able to always view at least a month in advance of the current date, because I stretched my To-Do Bar wide enough to display two months in the mini-calendar. Now, as the end of the month nears, I can only see a few days in advance. Sure, I can click through month-by-month with the arrows at the top (which is why this is the lowest priority of my three gripes), but it sure was nice when I could always glance ahead a few weeks and see what days were open (when we had the bolded indicator), or even just what day of the week a certain date would fall on, without having to click through to the ext month.