Here are the most fundamental errors:
The withering away of the state
The dictatorship of the proletariat
Concerning the former, the idea of grassroots democracy is great; however, the state needs to be the whole People (all the democrats) from the grassroots through the layer that ties all of the locales together. There was never anything wrong with having a state, per se. The only thing that ever mattered, and still matters, is the quality of the state: good government; good state; good People. In our case, we need good government by the self-governed, all the People.
Concerning the latter, democracy and dictatorship are incompatible. Dictatorships are inherently elitist and anti-reconciliation, anti-educational, anti-forgiveness, anti-mercy, anti-total-consensus building. It is only via total and complete consensus building via the utmost transparency and compassion and the like that the left will ever permanently succeed.