Few things are very important for GT - and those are the main differentiators from all other console sims:
- its physics is developed from-wheel-to-controller (since GT3) and not vice-versa. Result is equal physics for all types of input devices where controller players do not have usual advantage over wheel players (as on all other console driving games) and they can drive in the same race with equal precision and handicaps (it is the greatest part of Akihiko Tan's magic regarding physics of GT series IMO) which creates extremely competitive environment non-related to input device
- if you play with the wheel then you have the greatest wheel-FFB support of all console games. And since GT games have FFB support since 2001 (GT3) the userbase with wheels is vast compared to all other games. Thus, you get great chance to meet and race players who race it on wheel, thus more serious
- actual multiplayer was the prime focus of all GT games, even prior to online-era. From GT1 you had Memory Card battles as superior 2P mode of that time (backthen there was no game on consoles that allowed you to race tuned cars from garage VS friends). GT3 introduced 6P iLink connectivity, GT4 LAN 6P connectivity. Online MP options for GT5 are vast for a console game, only console game I know that have so much diverse options that cater serious and organized players is RacePro
- also regarding realism, I have to highlight maybe the most important part of realism when actual game-design is concerned - consequences of ownership/usage of cars. That particular design is the most unique and most important single aspect of GT series that was constantly developed and improved since 1997. Once you purchase the car and start using it, you are getting actual consequences of that usage. Engine wears-off (higher revving will accelerate both fuel-consuption and wear of the engine), engine-oil needs to be replaced with mileage, chassis wears-off with mileage and reckless driving (which changes suspension properties of the vehicle), aerodynamics properties are lessened with accumulated dirt, etc. All those consequences can be repaired in-game, but after some time (and mileage) car can't be returned to brand-new state (only with special Prize Cards which are very, very, very, very rare). Thus, you get actual consequences for driving a vehicle (mentioned repercussions appear in all game-modes except Arcade and online Free Run mode), which is very important part of the overall ownership philosophy of GT games. Also, this is one unique area where no other driving-games developer ever even tried to step-up and develop that area of their games. And that is one of the reasons I personally find GT series unique and worth praise because they are doing what noone else does in genre.
- GT5 also have one particularly interesting feature that appears on tracks with variable weather. Special sub-engine that is tied to weather engine is doing real-time calculations of on-track temperature/air humidity/air temperature/air pressure. Although those data are nowhere to be displayed in the game, they are the one that actually powers the weather engine and decides the possibility of rain/snow/blizzard/etc, when it will start, when it will stop, etc. I personally also find that a extremely important part of the overall realism-factor.
Many things that GT series does through time are really genre-groundbraking and unique. Of course, since there is no perfection in the universe, GT can be and should be criticized for its shortcomings, but it should also be praised for what is does great and different then others. It should not be matter of personal preference towards some aspect of the game, but objective verification of what GT does great. And I really think there are much more fields where competition can learn from GT, and not vice-versa (all I mentioned above fall into that category).
And unfortunately, we live in the times where some long-term qualities, unique game-designs and less-supraficial features of GT5 are criticized, overlooked or deliberately ignored in order to cater some subjective opinion or fuel neverending platform&title wars. While in the same time it would be much better if majority of shortcomings and omissions from other games were highlighted with the same aggression and effort as GT5's were - it would make those other games much better from perspective of serious racing and long-term enjoyment, which is unfortunately not the case.
