This is for a list of all the features that are included in 4.0 that aren't the obvious ones (multitasking...etc). Here are the ones that I have found while I was playing with 4.0...

-There are new fields that you can add for contacts. You can now associate an IM name with your contacts, with support for AIM, Yahoo! Messenger, MSN Messenger, ICQ, & Jabber. (Maybe official IM app later?) You can also add birthdays and anniversaries, which will then be added to calendar.

-You can also link contacts, but I tried this just playing around then when I went back to it to actually figure it out the option was gone.

-You can change the size of the font in the accessibility menu

-Smart playlists sync in the same order they are in iTunes!!!

Originally Posted by DSchnake09

There are new fields that you can add for contacts. You can now associate an IM name with your contacts, with support for AIM, Yahoo! Messenger, MSN Messenger, ICQ, & Jabber. (Maybe official IM app later?) You can also add birthdays and anniversaries, which will then be added to calendar.

Aside from adding to the calendar on birthday/anniversary you can do the other stuff with 3.x - maybe they have made it more obvious/easier to add other fields in contacts.