The '03 has the following "improvements"... which is debatable, as I wouldn't really complain too much if I was "forced" to ride a '02 and earlier.
High flow head. (About 5-6 more HP/TQ... depending on what you read)
Cam to accommodate the head.
Second counter-balancer on camshaft.
Larger diameter collector.
Elimination of the separate cross-over pipe.
Higher flow oil pumps.
Oil level sight glass.
Change in swingarm "beams".
Easier to service second filter. (This change occurred in mid '01)
Larger diameter frame filler bolt. (A wonderful thing... really)
10A un-switched powerlet plug in dash.
Fuse block relocated to battery area from fairing area... along with changes in wiring loom to accommodate this.
Tank decals beneath the clear coat... bastards.
'04 Adventure: (Not available in the US, unless you "grey'd" one in)
950 Adventure front fork, low fender, brakes... and kinda "soft" wheel rims.
Very snazzy orange paint.
'05 Adventure: (Approximately 200 imported into the US)
Brake pedal changed to a folding type.
Reduced fuel capacity. (About a gallon?)
Second oil jet to cool underside of piston.
Some kind of big honkin' air system that I haven't seen in person yet but I hear it's not too hard to remove.
Even snazzier orange paint.
That's all I got off the top of my head... and I could be wrong.
Anyone else got anything?