Either you haven't played enough or it's been too long since you played GTA IV
The damage system has been massively improved. I think the reason people often complain is because some big impacts don't result in huge deformation. I think they have purposely made car vs car collisions less damaging so that the car doesn't get wrecked every time you bump someone. And they have made the engine last a lot longer when taking damage so you won't catch on fire anymore after 5 or so big hits. I guess it's down to personal preference if you want more sensitive damage but given the size of the map I think it's much better to have a stronger car so you don't get stranded.
From what I have seen and experienced:
-The wheels are easier to damage now, one big impact to a front corner and the wheel will jam and the car is useless. Also wheels will fall off from a single big impact.
-Huge impacts will cause the car to explode on impact
-Cars leak fuel from gas tank after heavy damage
-Deformation looks a lot better than GTA IV
-Bumpers fall off
-Chassis get bent and the doors won't shut properly
-Even the lightest contact will shatter the windows of affected areas
-Engine damage causes car to misfire and backfire
And remember each car reacts entirely different to damage. The big old classic cars are very hard to damage compared to the more modern cars.
