Those worried about carbon emissions will say coal power is the main challenge, while the communities that live off of coal power and/or coal mining will say they need to be protected economically. A fast coal phase-out would be good for the climate, but some sectors of German society do not want it to be too fast for these communities and workers.
Those worried about air pollution might point to diesel and gasoline cars in addition to coal power. Over the past decade, the German government has blocked strict emissions regulations at the European Union level in Brussels that might have impacted German manufacturers of luxury cars in particular. Now, electric vehicles have the potential to make German expertise in diesel irrelevant. Some 800,000 jobs in the German automotive sector are at stake, easily twice as many as currently work in the renewables sector. Transitioning German carmakers over to electric vehicles is a major challenge, especially since they may not be leaders in this technology.
In the power sector, expanding the grid in order to reduce grid congestion is another challenge. The grid needs to be expanded anyway, but some German citizens and experts would like to have more of a focus on smaller, local community projects, not just a lot of wind turbines in the windy north, which then require power lines through the entire country to reach the energy intensive south.
And that brings us to social acceptance. It cannot be assumed that Germans will always accept the Energiewende, especially as its character changes. It began as a grassroots movement but has become big industry. Quite possibly, the Germans who are the most excited about renewables are already involved; we may have already picked the low-hanging fruit in terms of social acceptance. A lot of Germans still like to drive gas-guzzling cars, eat meat regularly, and other things that are not good for the climate. The challenge of getting everyone to change their behavior, even when there is no financial payback, maybe the greatest one of all.
But most of all, it is important to understand that the Energiewende is a complex undertaking that will span generations. Mistakes will be made along the way. The challenges cannot be simply listed in order of objective importance.