The cool thing about the address book is the it is integrated with Skype and VoIP accounts. However, one feature I miss from my S60 phone is the ability to call fax numbers.
The N900 assumes that a fax number is not dial-able. Unfortunately, I used fax numbers from my GMail Contacts for additional numbers. Now when I sync with Google Contacts, I can’t dial the fax number. There is already a bug created for this issue.
Another feature lacking is speed-dial. The phone app requires selection of the contact or complete number-entry.
Hopefully they will change this “limitation” in the production firmware.