In the early stages of the development of The Witcher there was a Reputation System planned as part of character development. Geralt could earn a good or bad reputation depending on his choice of action in the game. For the final version of the game this feature was dropped completely. Unfortunately there is not much known (yet) about how Geralt earned a good or bad reputation. One would assume that he earned it in the way how he responded to other NPCs. It is unlikely that quests alone were the source of good or bad "reputation points" because one of the prominent features of the game is that there is no differentiation between "good" or "bad" quests.
It stands to reason that CD PROJEKT RED refrained from using this feature precisely for this reason as it would suddenly be at odds with their statement that "there are no good or bad decisions, only choices".
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At any rate Geralt's reputation had an impact on his environment and, in particular, on the NPCs with whom he interacts. A good example is the diversity of bandits. If Geralt had a bad reputation it was easier for him to get information and even support from bandits. He could engage them to help him in several ways to complete any given mission when he was in need of a bit of extra backup. The game trailer about the decision system in the game offers one option to solve the quest: Geralt with a bad reputation makes a deal with thugs to help him. Of course Geralt with a bad reputation building a rapport with "the criminal element" should take care to avoid the City Guard who already have a poor view of witchers and probably would not hesitate to arrest him.
If the witcher had a good reputation, then he would be likely not to tangle with bandits. They would not help him, they would likely be even more hostile. But on the other hand Geralt probably would get support from the "good guys", the City Guard. Possibly this could have avoided bribes. Likely he would get more support and information from townsfolk and peasants as well.