Here is a round-up summarising a number of issues with the first beta.
(I'll link or create any associated bugs where appropriate, or point them out to me if you know), note some of these issues are known/stated issues:
No proxy or web access for zimlets
Bug 30313; support for proxying from Zimbra desktop
This has been discussed here, I believe: ZD2 and Proxy - so i seems it will be resolved around the next beta release.
Zimlets do not display in mail module
See bug: Bug 42575; Zimlets pane missing in accordion view in Mail application tab
Installed zimlets only display from other modules (such as addressbook, tasks etc...) preventing their usage for mail-orientated tasks.
Zimlets only work in relation to local folders
I have created a bug for this here: Bug 44577; Zimlets work only in relation to 'Local folders' (Zimbra Desktop 2)
The actions of Zimlets only apply to local folders, i.e. Birthday Calender reminder only creates a calender in local calenders, and only uses birthdays from local addressbook. This is particularly unhelpful. The user should be able to choose, or at least set the default account themselves, or else have an accordion-style display of zimlets at the bottom of each account tree. Vote if you want this changed quickly!
Zimlets are not deployed
In spite of following instructions in this thread: Deploying Zimlets in ZD2 Beta1 , zimlets were not deployed, I only managed to deploy them by creating a '_dev' folder in '~/.zdesktop/zimlets-deployed' and copying the unzipped folders into it. Of course, this is only intended for development purposes, and not for general usage.
It is hoped that by release of Zimbra Desktop 2 final (GA) this bug will be implemented and allow users to deploy zimlets via the UI (vote to encourage it!): Bug 31419; Deploy Zimlets in Desktop (UI)
Spell correction in html emails not functioning
When composing an html email, right-clicking a work marked as misspelled does not offer the spell-check menu, it offers only the formatting menu. (If opening a plain text email this works ok).
Spellcheck not functioning
There seems to be no way of doing a mandatory spellcheck.
Mandatory Spellcheck blocks sending of emails
If a user sets spellcheck to be mandatory (in preferences, mail composition) then they cannot send emails as the spellcheck will not compete.
I have created a new bug here: Bug 45085 When mandatory spell check is enabled outgoing mail cannot be sent.
Read receipts not functioning
I have created a new bug here: Bug 44574; Read receipt requests are not notified and cannot be accepted or declined
Messages with read receipt requests appear to be shaded differently, but they are not explicit, and do not appear to issue any notification or ability to accept/decline.
Default setting for responding to read-receipt requests is to refuse
The default settings is fairly well hidden in preferences, it should be better exposed, and it should be set as default to ask the user whether to accept or refuse, not just silently refuse, or most users will never know any better. The first time a user is asked for a read-receipt they could be requested to update the setting for future.
Cannot effectively set custom field in Contacts
I have created a new bug here: Bug 44575; User cannot enter a field name in custom contact field as cursor constantly exits
When setting a custom field in Contacts (under 'Other') a user can type in the field value easily enough, but when attempting to type in the field name, after entering each character the cursor jumps to the field value box!
The current release is stated not to be fully internationalised, so hopefully these internationalisation issues will be ironed-out by the final release:
Language reverts to US English
I've created a bug here: Bug 44579; Chosen language is not honoured and reverts to US English
Regardless of the language selected, the language reverts to US English (seemingly randomly, and especially, but not necessarily after a restart).
Lack of access to internationalised dictionaries or instructions
This would be addressed by this enhancement bug: Bug 35874: provide same way of installing/switching dictionaries as FF does - vote if you want it!
In the meantime, I have managed to install an internationalised dictionary (in my case for the en-GB), however I had to resort to using instructions and dictionaries which were linked from a Zimbra wiki page which is no longer available (I used the Google cache of the page). There needs to be a widget in preferences where the user can automatically download and install the necessary dictionary and update settings, from within preferences. There should be documentation at least in the wiki also.
Performance is greatly enhanced in ZD2, however the following suggestion might benefit some power users:
Lazy loading user-customisation
Vote if you want it! - I have created an enhancement bug here: Bug 44576; Lazy loading - user customisation
The user should have ability to turn-off lazy loading for those modules considered necessary - i.e. Contacts (Address book) if they are in constant usage. This facility would be in preferences with a warning about potential impact on overall performance.