Essentially that there is no way that the dictatorship of the proletarian will ever be temporary and that even though he spends time talking about the evils of the bourgeois state and how it must be dismantled this dictatorship with its "vanguard" (which are just new elites) will necessarily form a new state that will also form self-preservation methods and never transfer to a workers run stateless system. Bakunin calls them a red bureaucracy and Emma Goldman writes about how the Bolshevik state simply replaced the old Tsarist state and became reformist and bourgeois in nature losing its revolutionary character in time. The crackdown of the Kronstadt rebellion was the first seeds of this nearly immediately after the October revolution. The anarchist response and alternative to the centralized state that Lenin believes is required is a decentralized system of worker self-management pods in federated councils and communes, not a top-down elite vanguard run dictatorship.
A lot of the disagreement comes from the understanding and lessons learned from the Paris commune. Lenin believes this is a prototype of a worker's state with recallable delegates, less red tape/bureaucracy and the removal of the existing state but Anarchists don't believe the lesson here is to just create another state, although I personally would argue a dictatorship of the proletariat is preferable to the dictatorship of capital we currently live in, we want more communes that work together. We don't believe the revolutionary character of the commune went far enough to actually destroy the existing state and instead tried to recreate it on a smaller scale.