I copied this from another site where I asked the same question. Here's what was said:
*The DSC/ABS modules are stored in the passenger-side open space under the hood in the 2001 model. That forced the wires to go right under the engine from the brake master cylinder to the modules, causing some failures. BMW changed the locations of the modules from 2003 to prevent the problem.
*The 2002.5 and later cars have a front strut tower brace, rain-sensing wipers and auto-dimming rearview mirror as standard equipment.
*Then as others said the 2004 cars have a slightly longer trunk handle (coupe only) and LED brake lights (2003.5), which come on brighter under hard braking.
*In the '01 you get the narrower nav screen and no available Homelink. In the '04 you get wider nav screen and an available homelink option. Plus the rear trunk handle is slightly wider. Overall there were very little changes throughout the years.