From what Iv read so far...
Improved graphics engine
Improved driving physics
Rain and snow weather effects (not dynamic)
New side events like gatecrasher and demolition derby events
More details and images of all drivers in the game
New boss races introduced to the career? Race the top driver of the land
Possibly reduced finer details off driving setup and garage (Sponsorship, Mechanics etc)
I also think they hinted at re-recorded sounds for cars and co-drivers.
http://www.gamereactor.no/previews/144341/WRC+3/?page=2&sid=1cd84fad03720187a81a0dd9e6572a9c
New video -
Graphics are a huge improvement! Glad they finally changed the cockpits aswell. What's with the GPS track map at 4:54?
Disappointed that Great Orme is still in a forest instead of on that narrow coastal road on the side of a cliff. Didn't see any changes to Finland either.
Everything looks improved apart from the handling, which I really enjoyed in WRC2. Hopefully it was just a bad driver but rear grip looks about non existent.
Armindo Araujo at the end playing as himself.