My main point would be in usage aspects. When you look at the details of a contact, then do something else and later go to the phonebook again, it’s still in those details, not in the contacts list where you’d expect it to be. Also, the details screen wastes too much screen estate – you could easily put all the information on the screen at once, and add some color for readability (probably display it like a business card – now it’s more like the developer’s first draft after he pulled the fields from the database browser).
Another thing (which would need to be done in something like Outlook first) is linking of contacts. I may have different members of a family with their own phones, but they probably share a lot of the other data, like fixed line phone, fax and address. It would be nice to be able to link contacts to other contacts’ details like that, and just add stuff like mobile and birthday separately. Make that syncable online, and you have a killer app going. Add Plaxo and LinkedIn or Xing for added fun.