There are a couple of issues that we're having with the calendar.
A Calendar that isn't tied to tasks.
First it would be nice to be able to create a calendar that wasn't tied to tasks. Our organization would like to have a way to visualize things like work/vacation schedules, employee birthdays and anniversaries and, events to name a few. None of these really need tasks associated to them and we don't want to have to change their statuses or start building a long list of uncompleted tasks.
Custom Calendars should be editable to everyone it's shared with.
The other issue we see is with the Custom Calendar. Right now only the person who created the calendar can add to it or edit it, even if it's shared with the entire team. This is annoying as we want to work collaboratively and have different team members add their tasks to the calendar as well, but right now that is impossible.