First off, i repeat what others have said, don't compare this to Forza 2, which is a racing simulator. PGR4 improves in every aspect from PGR3, from gameplay,graphics and sound. It is a a solid racer, and despite being more of arcade than simulation, it is still very challenging with great handling.
Weather effects are a great addition, clear, light,heavy rain,wet,icy,snow,storm etc. Graphically the best racer on Xbox 360. The meaty engine sounds are great, music is nothing special, but some nice classical !
Photo mode allows you to upload/save your photo creations on PGR Demand for other racers to view and vote for.
Simply a great game