Photorealistic bumber cars being GT i presumed and Cartoony damage being Forza.
If you play GT4 the "right way" you don´t have bumper races, you don´t cheat you dont race in over powered super cars against Ford-T models. As someone likes to say over and over again, consumer choice, sorry it´s a player choice.
If you pick the "right way" the experience is really mindblowing
Not to mention the penalties for hitting walls and going off-road has been increased significantly in GT5. Ever tried to "bumper" your way around a corner in GT5? Your car would rattle profusely, like in real life.
My personal opinion is that damage modelling might detract from the original GT spirit. To me, the GT core has always been about forever perfecting your racing lines, corner approaches, fine throttle control and such, ultimately to beat your PB or even the WR out there. Crashes don't necessarily encourage best line driving, as there are already realistic physical penalties for hitting obstacles or driving on grass. If you had a catastrophic crash, you would hit "restart" anyway, as anyone would.
GT has been successful in simply being itself, and for now, does not seem preoccupied with a feature-checklist-ticking contest with other wannabies. I am personally glad about this.