1) They underestimate problems with their ideas, and then brush them off with solutions which, when summed together, make the visionary idea much less palatable. For example, the idea that we can abandon traffic lights and stop signs is complicated by the fact that not everybody can, will, or even should use a car to get around. But in the world without traffic lights and stop signs, pedestrians and cyclists are trapped by fear. Sure, they might technically be able to walk out into traffic and the wondercars will stop for them, but in reality, nobody will do that, because the cost of being wrong is death. In every conversation I have had with these technophiles, the solution always ends up being some massively costly network of pedestrian bridges at every intersection of the city. Yeah, not so palatable anymore.
2) They fail to understand that, like evolution, intermediate states have to be viable improvements on their own merits. Wheeled animals do not exist, not because wheeled animals wouldn't have superior mobility, but because no possible intermediate evolutionary state has incremental benefit over the previous legged state. How do you transition a 2 lane road into three lanes when only 50% of the cars are self-driving? What about when 99.9% are self-driving? There are thousands of imaginary future states that do not have credible intermediate states, and will likely never see the future state as a result.