There are a hell lot of changes, both Above the hood and under the hood. There are not just big and huge changes, but small and minor changes which affect the daily use a lot.
You have an active standby in all the S60v3 phone, although the last few s60v2 phones did have this active standy. It display your calender entries, some phones display your email and messages, the newer phones even notify you about missed calls and all.
It has a browser based on safari code, gives you a desktop like browsing experience. With Mini Map, mouse cursor, rss feeds etc.
There also small small changes like, after you come to the end of the list, like the last mess in the inbox, if you still move the joystick downwards, you will move to the first mess in the list, in the previous S60v2 which i have tried, you had to move to the top message one by one.
S60v3 also sports a much enhanced platform security, breaching which is extremely remote and virtually impossible task.