Yep, it's pretty much boring for me too. GT3 was never boring. I played that game, over, and over, and over again every time I finished it. That game was fun to start over.
Currently, I'm at 75% finished, and I don't care to continue. I might want to get a few more cars, like the Speed 8, and some others you can't win like a few Supra and NSX JGTC race cars, but there isn't much left I want.
The races I play most are the Japanese Fuji 2005GT JGTC 500 style races. I often race my slightly race-mod Corvette Grand Sport with 396HP, and spank the hell out of the JGTC 500 cars. I also like to use GT300 JGTC cars with this race. I also like the 246 race with these types of cars, too. I also race in the Dream car championship races, from time-to-time.
The tracks I race most on are RR5, 246, Infineon, Circuit de le Sarthe, Fuji 2005GT, Nurb and Grand Valley East for smaller cars. Sometimes, I'll race on Cote d'Azure, or a few other courses, but not that often. I never run the Rally courses anymore. Well over half the races I run are Family Cup. That way, I can choose the difficulty level of my opponents and race against cars that are much faster than me.
For me, this game just doesn't flow right. There's something missing to this game that GT2 and GT3 had that made me want to play them over, and over again once I finished them. Sometimes, the races feel more like doing "chores" than having fun. Plus, the Rally Races are beyond stupid thanks to the penalty system and difficulty controlling the cars with a DS2. The missions were interesting, but once you completed them, there was no need to do them again. Of course, the race events sucked.
Well, I hope GT5 is better. I doesn't need to be perfect, but different and better. Maybe some more JGTC 500 and 300 cars, Speed Challenge Touring Cars and GT cars, DTM cars, Grand Touring Cars, GT2 and LMP1 cars. We also need to see the 63 Cobra Coupe Race Car again. Oh, and much better race events.