So far, the interface is very nice and clean. The few people I have working in the interface have very few issues with it. It is, of course, only being tested now. I am sure more stuff would show up in production.
The only things users would like to see so far are:
Calendaring (minor issue)- if you have an appointment for 15 minutes, you cannot double click to open it, or mouse over it (tested in Firefox, not IE).
Inbox - Could use marking. would like to be able to select multiple messages and mark them as read or unread. Tagging is very nice for sorting, but the read/unread piece would also be very nice.
Contacts - I am sure when this gets to working with Outlook, this issue will disappear. Every user so far has asked for an ability to import froma file such as a .pst file or a delimited text file as most users have tons of contacts in Outlook or on their blackberry devices (another day).