This sort of discredits all your other points.
If anything, GT4 set the benchmark. GT5 is a half-arsed attempt that should have been called GT-Online since they completely forgot to ship it with a decent single player career mode. And then we don't even go into all the other game-breaking design flaws PD introduced this time around.
GT4 did set the benchmark, it was a big leap forward. However, GT5 improved on many different aspects and the driving model came along leaps and bounds, online was brought in which was a big part of it. Career mode wise some people dislike the format but it really has not changed since GT4, in that GT4 had no more of a career mode than GT5, aside from more single events, while GT5 has had a large ammount of seasonals. I find GT5 is a game that most people will never likely complete, and its ok being that way. GT5 (or any GT game) is the type of game you can just pickup and play for a bit, put it down and come back 2 months later and keep working at it. The driving is where the enjoyment is had.
On topic:
Shift 2 is a completely different type of game to GT5, it is much smaller and much less advanced. It is a cool game and really should go alongside GT5 rather than compete with it. I am never going to play Shift 2 for as long as i play GT5, Shift 2 is simply smaller, it lacks the depth and the quality of the driving model is too far off to keep me interested.
On the PC the graphics for Shift 2 are frankly... amazing, the best i have seen most likely, while the car models do not rival GT5 premiums they are good. But the track quality (design/accuracy wise) is not as good as GT5, real world tracks are slightly innacurate and far too wide, Nordschleife feels massive it is so wide in this game.
The game for me lacks depth, is innacurate in many ways, adds too many fake elements (Like losing colour vision when i bump another car) and over dramatises the racing in some cases.
It is a great pick up and go game, i'm nearly 60% through now and i have had fun, raced on a bunch of tracks and enjoyed myself, but once i reach 100% its going to be a choice of running the inferior physics model at the inferior version of Nordschleife in Shift 2 or going back to the much more accurate and articulate opposition in GT5, the clear choice is i will go back to GT5 for my racing when i am done.
Biggest game design type annoyance for me is the "Best Line" - Getting experience for following what the game decides is the best line, encouraging you to use the driving line and honestly making you slower. The line just isn't the best line most of the time as such a thing is massively car and situation dependant and forcing you to use the driving line aid in order to level up faster is a flaw that has annoyed me.