BedZed is an ambitious mixed use development created in England in 2003 that was intended to have zero carbon emissions. To some degree it has achieved its goals. Energy and water use are down considerably compared to a conventional house, but no where near its initial goal of being resource neutral (not needing any outside water or electricity). Despite is accomplishments, a lot of features have not worked out as planned, and should send a cautionary note to those who want to experiment with bleeding edge designs.
As to its downsides I find the architecture to be cold and impersonal, like a giant factory. People want to live in buildings that are attractive and cozy, and Bedzed is no that. This is a very common problem with architecture these days where ugly designs predominate. Aside from that, the on site independent biomass generator, waste water treatment system and rain collection system have all been abandoned because they were too difficult and costly to maintain. The roof can leak and having plants growing on top makes it much harder to fix. The solar heating system generally is not strong enough in winter and tends to over heat in summer. People complain about insufficient parking. A scheme to get residents on electric vehicles has totally failed. The solar panels are largely a failure due to the climate and angle they are located on. Some people complain about a lack of privacy.
Despite some of its successes BezZed has a long list of failures. In fact the only thing that really has worked is the insulation and the passive heating, which brings with it its own problems. These projects are announced with great fanfare and triumph, and then reality sets in. Of course, I suppose that we should appreciate that they are willing to take risks and try new things. They could have done BedZed bit by bit learning as they went on, but they wanted to do it all at once. Unfortunately when it goes wrong it is the resident who really suffer, not the architect. The lesson is that those who take the leap on new visionary but untried systems are likely to end up as Guinea Pigs.
Here is a link to an assessment of the good and bad of BedZed.