Short answer
(1) The desired pace of progress and (2) views on minorities.
Longer answer
Both Libs and Communists want to proactively change the way we’ve been doing things, our culture. In their heads, this also will make humans better, wiser, kinder to each other. This is why they both are progressives.
Communist hares and liberal turtles
However, there’s a huge difference between the two of them. Communists want to improve the world in a giant leap, by abolishing private property. Meanwhile, Libs have more faith in incremental changes. In a way, Libs are “Kaizen Progressives”.
This is theory. In real life, from the right half of the ideological continuum—especially if you belong to the Trumpists, paleocons, right libertarians, hardcore Evangelists and alt-righters—it’s often hard to tell them apart.
But don’t despair! I’ll teach you a trick how to do it, solidly tested by us, the old practitioners of Soviet propaganda.
Identity politics
Communists don’t believe in identity politics. They believe in classes. The question is, do you possess the means of production—which makes it possible to exploit other people? If yes, you’re an enemy. If no, you’re a good guy, maybe one of us.
As to “identities” that people might stick to you, it’s a figment of bourgeois mindset. It’s there to confuse you. “Identities” distract you from your fundamental class association with the working class and your historic mission of defeating Capitalism and abolishing private property.
As Lenin said:
“People always have been the foolish victims of deception and self-deception in politics, and they always will be, until they have learned to seek out the interests of some class or other behind all moral, religious, political and social phrases, declarations and promises.”
Tactics
This is why Communists hate organized minorities—if it’s not the Communists themselves who organize them. It happens that Communists strike tactical alliances with the most influential and active minorities, like leftist libertarians. But these rarely last long. And they never survive Communist power grabs.
“Minority” itself is a dirty concept for practicing Communists. They are passionate “99 percenters”. There’s strength in numbers for them. The central point to their faith in themselves as a vanguard of the working masses is their ability to lead the proletarian majority (not necessarily electoral one) against the tiny clique of exploiters and bloodsuckers.
Conclusion
Look at their view on “identity politics”. No matter how far to the left they go, if they encapsulate their supporters within a narrow cocoon of shared lifestyle, or ethnic and cultural values—they are not Communists.
Below, some examples of visual propaganda that may help you tell Communists from the rest of the progressivist pack. First, a drawing of Gay Pride parade shared with me by Stephen F. Condren. Through the lens of Communism, this is a patently parochial, ideologically wobbly, Liberal approach to politics, revealing a “confused, petty-bourgeois mind”:
As a contrast, look at the Soviet propaganda poster below. It’s dedicated to the Olympic games in Moscow in 1980. The Olympic colors making the teddy bear’s belt and the word MИР (“peace”) symbolize human diversity, just like the rainbow flag above.
However, the Communist teddy bear Mísha to the right refuses to let the diversity run its own course. The flag in his paw is distinctly red. The Olympic symbol on the flag mimics a tower of the Kremlin, the beating heart of world Communism. It has the same golden color as the hammer and sickle on the USSR’s banner. The flag is raised high above the cacophony of apolitical colors. There’s no doubt who runs the show.
The text says: “Our main motto is peace”.
Below, a Stalinist poster from the early 1950s, “We won’t allow to sow discord between peoples of the world!” Variations of this motif with small additions used to adorn many a gay club in Moscow in the 1990s. The stern faces, close body contact and a distinct sense of hierarchy reveal a clear-eyed Communist mind behind this piece of progressivist art.