If the damage is already present when the car is collected and is logged on the appropriate paperwork, then the damage should have no effect on the renter whatsoever. If there is damage not logged on the appropriate paperwork, then the renter should inform the rental agency of it before leaving the site, to avoid being held liable for the damage.
If the car is returned with "new" damage - either because it wasn't previously noticed or has now occured, the exact consequences depend on the rental company's policy and the rental agreement, as well as the insurance held by the renter. In general, the renter can be expect to charged for the damage, at what many would consider a somewhat inflated rate. The requirement of a credit card when renting is normally to make it easy for the rental company to do this. If the renter opted to reduce the damage collision waiver to 0, then this should cover any non-malicous damage (you may be questioned about the circumstances) and there should be no additional charge.
Note that many car hire companies exclude certain aspects of the car from their damage insurance/CDW. The most common exclusions are tyres (sometimes all aspects of the wheel) and windscreens. Also note that letting a non-named driver drive, or operating in any of the (often many) prohibited situations of the rental agreement can also invalidate any cover you may have.
With respect to the impact and reaction to the size of the damage, it's really difficult to generalize. Even among the big chains, rules may differe from country to country, and possibly between chains. Some have a policy on a minimum damage size before charging, others don't. If the damage was pointed out when renting the car, then that is probably because it is chargeable damage (those rental agencies I've used with a minimum damage size don't bother to point out marks below the minimum). Whether damage gets fixed or not is not an indicator of if you will be charged or not. This is for several reasons:
- The rental company will argue that the damage effects the resale value of the car and hence the value of their asset
- Cosmetic damage may only be fixed periodically, especially in busy periods when all the fleet is used
- Car damage and subsequent charges to customers are a profit centre for many car rental companies. This is more common with companies in heavy tourist areas for whom repeat business is a smallr concern than low prices to get new business and maximizing the profit from one-time customers.
I've also seen cars being rented out with quite significant (ie major dents), cosmetic damage before, which it is almost certain was charged for.