The Pro version also has more gauges and some more detailed cities.
The extra cities are nice to have, but the gauges will probably become almost essential once larger numbers of 3rd party aircraft become available.
Cessna Grand Caravan
multiplayer instructor station. A remote computer can be set up to monitor the flight (and I think also to influence some things).
With FS2000, most of the better aircraft required the Pro edition to work correctly.
get the full laydown on: http://www.microsoft.com/games/fs2002/features.asp