The main result of not exposing a dog to certain stimulus is fearfulness and thus the potential for aggression (fight) because they have not been afforded the opportunity to develop coping skills. However, fearfulness can be debilitating in other ways, like dogs that are nervous and hide all the time and cower in a corner (flight and avoidance).
Fearful dogs can also develop eating disorders and other nervous issues like self-mutilation and excessive barking. These fears may also develop into phobias – but the over-riding problem is one of fearfulness and this of course will result in behavioural problems for the owner which may require experienced help to overcome when it could generally have been totally avoidable. The longer we leave exposing a dog to certain stimulus, the more fearful they become and we can struggle to help them at a later stage because they lose curiosity due to this fearfulness.
Dogs that fail to develop a knowledge of certain stimulus and thus fail to develop coping skills are the ones that are more likely to go on to develop phobias later on. More specifically they are the dogs that are more likely to develop non-specific anxieties and this makes their problems harder to fix.