Re: Still waiting
I never saw why a mail client should integrate tasks and calendars, or for that matter contacts...
Contacts at the very least should be in their own program, with an easily called API that allows other programs to make use of them. The idea of having an integrated contact list in the email client dates from the days when email was pretty much the only communication medium used on computers... These days we have IM, VoIP and various other services so it makes a lot of sense to have a centralised contact list.
A calendar is also not related to email, and should not be in the mail client... I prefer the Apple approach where separate applications are provided and you can use whichever you want.
As for your statement "in an office/exchange way", this is down to exchange using proprietary protocols to make it difficult for third party clients to communicate with them. Outlook also doesn't support any standard calendaring protocol, and only has very crude support for IMAP mail.
If you have a standard (CalDAV) calendar server, combined with a standard (CardDAV) contacts server and LDAP for directory lookups, plus standard IMAP mail there is a lot more you can do...
You could use the Mozilla suite (thunderbird, lightning), the Apple suite (iCal, Mail.app, Addressbook) etc.. You can even use Outlook, but you'd need a third party plugin.
Because the protocols are standard, writing your own tools to interface with the system is much easier, for instance you might have a job or bug tracking system that automatically populates your calendar with time allocated to work on particular tasks, and you can use other metadata (eg presence of your user on the vpn, presence of your assigned laptop/phone in the office network) to work out where you are.
Also because the interfaces are standard, you can switch implementation of the backend server as it suits you without breaking your custom apps.