“It’s a good idea in theory.” No, if an idea is good in theory it should at least be plausible. Communism is a good fantasy in theory. Communism completely disregards human nature and requires social engineering, brainwashing, censorship, and enforcement by state violence to even resemble a working system in practice. The adoption of a classless society has never actually happened, not even in countries which claim to be communist. A classless society would mean socio-economic equality for all its members. This is completely ridiculous, as neither people nor their labor are equal.
Communists think that people will behave altruistically towards one another and be devoid of ambition or rational self-interest, both of which would encourage individuals to increase their wealth or standing (which almost always comes at the expense of another’s wealth or standing). We cannot all be rich. We cannot also all be poor either, since even in communist countries the “proletariat” ends up being ruled and directed by elite party members who have amassed titles, offices, revenues, and properties. They form an aristocracy rooted in nothing but their adherence to Marxist dogma and inspire no loyalty. This is why communists always focus on the middle classes, the bourgeoisie, as the enemy that obstructs the class struggle and prevents the working classes from reaping the fruits of their labor. That the capitalist is hated is a given, but special hatred is reserved for the middle class, who while not being rich themselves, often align with the interests of the rich. They are the easiest target and distract from the Marxist aristocracy.
Thus, their hatred and envy, not only of those on top, but anyone above the lowest-common-denominator pool of the masses, pervades society. This, combined with the dehumanized view of people that emerges in communist societies—where numbers are continuously crunched, plans developed, but profits never made—of any means being necessary to achieve the classless end, results in the archetypical mass destruction of human life being seen as a necessary sacrifice for an intangible and untenable goal. Hatred and envy of their superiors married to delusions of egalitarianism result in tyranny of the minority, on behalf of the majority, against everyone.
Many arguments against communism also apply to leftists in general, who having been influenced by Marxist thinkers, essentially advocate for the road to communism rather than communism itself, i.e. socialism. Socialism doesn’t cope with inequality but patches it and allows the problems to persist from their sources. Communism and Fascism are much more interested in attacking those sources, but from very very very different perspectives.