The lack of a good career mode in GT3/GT4 is something that is really missing from the series... IMO the career mode was even better in GT3 than GT4... you should start out as a racing rookie and build your career. Learning how to adjust all the settings to your driving style etc. GT3 made that good, but then they just seemed to focus on the racing sim bit in GT4.
I might be mistaken but I felt that GT3 was a more fun game than GT4, but perhaps my expectations just was too high. Which brings us to GT5. It has to perform _really_ great to live up to the expectations after that long of a wait. The PS3 is already dead to me. I waited for GT5, I got bored in the meantime and sold the console.