Year differences between tiptronics
All the tips until 2001 are basically the same on a 996. The 2002 model saw Porsche use the Turbo tiptronic unit instead of the older one. This gives 2002 cars a 'stronger' tiptronic and some other features.

Manual shift in D
When in D, you can still use the buttons to manually shift. It returns to D after 8 seconds in a gear.

Stationary overrev protection
You will not be able to rev the car while it's stationary beyond 4k RPMs. Don't do this anyway or you'll be buying a new torque convertor (2k installed).

More shift patterns in D
It has 250 shift patterns instead of the 5 or so on the older box. This means it can be cleverer in D.

More robust tranny
The 2002 onwards has a transmission from a much more powerful car, the Turbo. So, I'd imagine in a normal 996 that it will last longer than a pre 2002 transmission would.