After having used the demo for just a few minutes I thought I'd post my thoughts.
1. When adding a new contact I expect my cursor to move from left to right top to bottom when I press the tab key. Currently the form goes from top to bottom left to right.
1a. In case the developers have their herts set on top to bottom left to right it would be good to highlight each field with CSS as I change the focus. That way my eyes won't be forced to look for a small blinking cursor. Maybe a 2px orange border on the field or change the background.
2. The "File As" drop down should work just like a regular drop down. That is, when I press tab it should be highlighted so that I can press up and down to change the option. When I pressed tab I expected for the drop down box to open up or be highlighted or something but instead the cursor skipped down to the E-mail box. If you had implemented 1a I would have immediately known my cursor skipped the File As drop down and went straight to the E-mail field.
3. Make up your mind. Is it Email or E-mail? My vote is for Email and even Google corrects e-mail to email.
The email section is titled "Email" while each field within that section is titled "E-mail".
4. In the Email view the New button on the taskbar has a line between the word New and the arrow pointing down. However, the Tag button does not have a line between the word Tag and the arrow pointing down. A bit inconsistent. Same goes for the other sections as well as the button called View that is found to the left of the task bar (and to the right of the section title).
5. I really don't like the embossed brush metal theme. And the embossed Zimbra logo looks ugly too. How is this usability related? The nicer things look the more likely people are to actually using something even if it's a piece of crap (which so far Zimbra is not).
6. It'd be nice to be able to adjust the left pane width. The reason I say that is because I am seeing horizontal scrollbars now that the vertical scrollbars have appeared (I've changed to the Calendar view).
7. Does the helps bubbles that appear over the icons on the far left really need to say "Go to Mail" "Show All Contacts" "Go to Calendar"? How about just "Mail" "Contacts" and "Calendar" (and "Help" and "Options")?
Isn't it obvious that by clicking one of those icons I will be taken to that function?
Anyway I've spent enough time with this at this point so I'll have to stop here. I hope these things are helpful.
So far I'm really liking Zimbra. The main thing I think looks cool is it's ability to recognize patterns (PO numbers, tracking numbers, etc.) and provide information about the matched pattern.