A 3 gig download for this when you already have gt5 doesn't quite seem worth it. Why couldn't it have been part of a patch too?
Because it is pretty much a different code altogether.
Graphics of GTAcc2012 are much more like in Prologue - sharper, crispier and with more polygons on both cars and surroundings than in GT5. Also, shadows are vectors, not pixels, so we're pretty much talking about the variation of the graphical engine compared to GT5 (in the line with saturation of Prologue - notice the crispyness of the Rome, especially lack of tearing and clarity of the fences and overall clarity of interiors and *ray-tracing* emulators for reflections on chassis, also not in GT5 but present in Prologue).
Than, you have specific new in-game animations and movies and completely new functionality of dedicated Leaderboards which is probably also another type of online coding and access to separate servers.
Than you have slightly different execution of force-feedback which could also imply modifications in the physics engine that will probably make a way to GT5 with one of the upcoming updates (whenever that happen).
Last but not least, few new minor functionalities has been added to the game which does not exist in GT5 at this moment:
- ability to change Race BGM/Race Effects BGM through "PAUSE" menu during the race (finally, 15 years of waiting for GT series
)
- filtering the Leaderboards
- real-time statistics
Of course, there is also a reason to have much easier stability when such application is executed as standalone donwload which is 100% same for everyone worldwide, especially for tracking the results without adding *weight* to GT5 online communication with the servers and much easier execution of the upcoming GTAcc patches and updates.
... 
