Personally, I like N900 as an Internet device, but the phone/contacts seems to be a bit lacking.

Besides the things already previously mentioned (contact groups, profiles with custom ring tone / ring group settings, contact/group-based ringtone setting, perhaps the ability of automatically change profiles based on time or calendar entries), I'd also like to see few other related features:

• The possibility of also defining special IM permissions to profiles ("only IM/SMS messages from the members in groups VIP + Work will play a tone")
  
• "Smart groups" that are automatically populated & updated based on one-or-more user-defined criteria. For example, group Work could contain all people where Company contains "Example Ltd"
  
• Contact/group-based IM/SMS ring tones