No Visual Feedback when Synchronizing
There's no visual cues in the interface to show that the calendar is performing a synchronization task–you click Synchronize and nothing apparent happens–nor does it show that editing your calendar while this is occuring could be a bad idea due to potential conflicts.
The first part of this issue could be addressed by displaying a process-working indicator (spinner) while the sync is happening and the second could be addressed by making the relevant calendar(s) insensitive while the sync process is going on.