"But now, I can think of a few criticisms of your point about dictatorship of the proletariat, which is to say, is it not true that, instead of the proletariat being in power in a "dictatorship" (the proletariat numbers in the millions), it was a narrow group around the leader himself?"
It wasn't really a 'narrow group', the CP of the USSR and elsewhere were massive massive organizations. The Party Congress elected the Central Committee, which itself was always a large organization about the size of our US Congress, and the Central Committee elected the Political Bureau. The Political Bureau itself was usually a committee of about 15 top administrators. The fiction of one person ruling a socialist country with an iron fist is just that, a fiction. I fail to see what the problem with electing representatives to administrate a country is. These representatives were elected from among the workers and peasants themselves. You're not going to have every single person in a 150 million person country be an administrator, and to say that is not to suggest that the country is not truly ruled by the proletariat.
As for people on the left heaping abuse on the socialist countries, yes - there are plenty. Mainly Trotskyists or similar types who claim that "Stalinism" ruled in all of Eastern Europe and this was a bastardized bureaucratic & despotic version of Marx and Lenin's pure vision. (Which is a joke since Lenin agreed with Trotsky about absolutely nothing). Trotskyist's claim that there should have been a world revolution (or at least one that spread to the most developed countries in the world), that socialism in one country always fails. They argue that the socialist countries suffered from allying with the peasants, and that this was a fatal mistake since the latter are too backward in their thinking to build socialism. They claim that the socialist countries were not really socialist, but "state capitalism", exploiting the workers just as the same as any capitalist country did. They will claim the leadership cadres became hopelessly corrupt, living pose lifestyles while the workers toiled. They will claim the workers saw this and became cynical about the whole project of socialism, and increasingly revolted against the political class - ala Czechoslovakia in 1968, or Poland in 1953, or Hungary in 1956, and the fallout from this was yet more repression.
In short, the argument is that while revolutions started out well enough, they were betrayed by power hungry leaders, who were more interested in their own power than spreading revolution and building socialism.
Some Maoists also argue that the East European countries degenerated, because they forgot about the importance of continual revolution. They forgot about energizing the masses with mass campaigns. They became complacent and content with living comfortable lives. They neglected their duties in fighting the remnants of the old ruling class, and so they became just like the old ruling class.