I haven't seen any information on what to do in shared spaces
The necessary information is there, namely the 20mph signs and visual/tactile demarcation between the roadway and pedestrian only sections. These are the only rules that you are obliged to abide by, the rest is supposed to be intuitive and down to the driver to subconciously interpret how they should behave in that area.
It's worth pointing out that there are a number of places where shared space has already happened to a degree without being designed specifically for that. Two examples come immediately to mind. One you're likely to know and the other probably less so.
The one that you are likely to know is Covent Garden. As Southampton Street approaches from the south you'll notice a distinct change in character as it reaches the main area. The surface changes, the kerb diminishes and although there is a barrier it serves more to ensure the sanctity of the no parking regulation than to prevent the unapproved appropriation of car-free space. That's probably a function more of the continuous train of deliveries rather than an expectation that drivers are hell bent on hegemonising everything in site as a driving place. The road continues in the vein until it reaches the end of the main space on Henrietta Street and through all of this space I think it's well accepted that pedestrians will utilise the road space at any time for any reason. It's well bedded in to the local conciousness that any new driver will still more or less abide by the local rules of the place. I'll concede that the barriers aren't wholly conduicive to the free flow of pedestrian traffic, but foot is still more dominant than wheel in the area.
The other one is the Woodshaw Estate in Royal Wootton Bassett. The kerbs and demarcation between road space and pedestrian are quite strong in this place, but the rest of the environment is pretty soft and the near complete absence of road markings and any long lines of sight in a residential area mean that vehicular traffic usually caters very well to what is mostly the dominance of the pedestrian. There's no expectation that people will cross the road at a certain place/time and wait for their turn. Both of these places work pretty well as share space with little to no objection from anyone.
There will always be some drivers that abuse the nature of the place that they are in but I think that the once a local character has been established this will pretty much only be the ones that would also abuse traditional road places. How we deal with these people in society is a different, broader matter, that needs addressing beyond just how they act in one small place.
As for why people moan, I think that once we establish a functional pattern of behaviour, no matter how well or safely it works, we hold tightly to what we know. The reaction to what is quite a significant change will, in some form often follow the five stages of grief and I can see that happening here.