The reference system used by Couchsurfing™ may seem simple and intuitive on the surface, but it fails to do its job. People read references for two main reasons: to make sure their experience is 1) safe and 2) enjoyable. However, given the way the reference system is structured, it doesn't achieve either of these goals and so the system is meaningless.
In general, a good user account is one that only has positive reviews. Negative (and previously neutral) reviews are seen as red flags. The norm is to leave a good review, which is understandable, especially in situations where a host has generously let you stay in their home and treated you well. However, there's a problem here in that it's hard to distinguish between an average experience with someone just letting you crash at their place or an amazing experience with someone who's a great asset to the community. As most references are similarly positive, it's very hard to gauge whether you think you'll have an enjoyable experience with another person. But the larger problem is in how uncommon negative reviews are. What this has done is consolidated members onto the 100% positive bracket whether they are great contributors to the community or detract from it. There's no way to tell from references.
The threshold for how bad an experience must be for someone to justify leaving a bad review is far too high, and so the vast majority of negative experiences are never reported, and so do not factor into the public references. People are confused about when to write negative reviews, especially in situations where they are made to feel uncomfortable. In many situations, they might not leave a review, or sometimes will even leave a good one. There is encouragement for people to use negative or neutral reviews more, but incentives were never aligned to make them commonly used. The topic of the reference system being broken comes up frequently, and has led people to not trust reviews, even if someone has many. This is an issue as it means that the reference system does not make people accountable.
Negative reviews are seen as a big punishment, and people feel uncomfortable punishing others. Often in these cases, a negative review seems disproportionate to a problem that might not have been 'that bad' (for instance, they were made to feel a bit uncomfortable). Negative references are stigmatized. It is the norm for users to have 100% positive reviews, any lower and it is much harder to meet people. There is a huge difference between 100% and 98% positive reviews. However, if people's scores were normally in the 60-80% region, then giving a negative review wouldn't be that big of a deal. There's not that much of a difference between 75% and 73%.
Although Couchsurfing™ doesn't publish references until both parties have made them, it is still intimidating to leave a negative public record. The stigmatization of negative reviews stops many people from writing one out of fear of confrontation or retaliation if they see that person again, or if they themselves are left a negative personal reference in response. There need to be anonymous ways to signal a bad experience.