1! I'm disabled and must use speech recognition which should never be confused with text-to-speech because it makes the disabled person more cranky than they already are. Zimbra is a great example of software created by developers that appear to hate disabled folks. the layout of the UI is not variable in size, there is apparently no API so that speech recognition and text-to-speech based interfaces can create their own UI, the visual indicators are sufficiently vague that it makes it difficult for a person with normal eyesight to see what's changing (read versus not read), and related to the scale of the UI, the various targets for doing things are very small making it difficult for someone with mobility impairments to accurately consistently hit the right target.
2! Calendar is very frustrating because I need to link with other calendars with full bidirectional synchronization and it hints it's possible with caldav but I find these partial hints in the forums and other things like that over and over again for the past six years and yet there's no definitive word? On the other hand, Google isn't all that forthcoming on their own caldav Implementation but still, don't be evil.
3) calendar is also very frustrating because I need to partition my calendars according to personal and professional needs. I have one calendar layer per customer because if they need to see my calendar, all I want them to see is my busy/free layer and their calendar events.
4) control panel layout is very confusing at times and wonderfully intuitive in others. For example, the refresh rate for various services when polling externally really belongs with the account using it. I spent four hours yesterday looking for something and I probably stumbled across it twice but by that time I was so frustrated I couldn't see straight and missed it. I knew it was there but I never quite found it even when searching with various keywords etc.
5) account inclusion. I've been struggling try to include an IMAP account from a client on the desktop and it's just not working. This is something that Thunderbird solved moons ago.
6) Why are you building a new multiprotocol client. Why aren't you fixing up Thunderbird which is just fine in many ways. Yes it still sucks for the disabled but it's known, it works and it also has a calendar and task management system on the side. Create plug-ins for briefcase and social and you are done. Seriously, it's got a much better UI, it's familiar to more people and it does a lot of things right. You don't need to show people how big your technical manhood is. You've already done a fantastic job with the server. That's good enough and recycling open source projects that need some TLC is even better karma.
7) on the other hand the Web client is pretty darn good. All it needs now is the ability to work with multiple accounts at once. If I had my druthers, I would ditch the desktop, use a Web client only that can work with multiple e-mail accounts at the same time and include non-zimbra residents accounts as independent standalone accounts, not the inclusion thing you use which is such an abomination I don't even want to think about it.
8) community support is thin. Either the product is so reliable that nobody bothers to check into the forums or is suffering the general malaise of nobody can afford to give their time away. I would love to see a community plus low cost pay for option for support if answers that work get recorded so that question doesn't need to be asked again.
So today I think I'm going to delete the zimbra desktop off of my machine so it won't torture me anymore with hints of wonderful goodness and work strictly with Thunderbird and lightning with syncing to Google. Zimbra is just going to be my e-mail platform until these other things get fixed. all that's left is figuring out how to tell Thunderbird where to put spam when you press the junk button.