I don't want to stop anyone from writing their own answer, but this response to a similar question by u/semiconductress is one of the better stabs at the question that I've seen.

EDIT: The rest of this comment was probably a mistake. I'm leaving it up so that people can see what everyone else is replying to, but... take the rest of this comment with a large grain of salt.

Side note: I was just talking last night with a friend who studies German and Russian utopianism and Marxism in that period, and coincidentally, she mentioned some things that now make me want to nitpick that answer's treatment of Rosa Luxemburg. For one, even as a revolutionist, she was in favor of electoral participation and women's suffrage in bourgeois society, and for another she was very opposed to Lenin's position, laid out in What is to be done?, that the spontaneity of the masses in a potential revolution would not be enough and that a strong vanguard party would be needed. [EDIT: I'm leaving that previous sentence in because I wrote it and I don't want to lie about my mistakes, but it exaggerates Lenin's pessimism. See the comment responding to me below, which I don't entirely agree with, but it's a good corrective.] So it's not quite right to say that they had "no fundamental doctrinal differences," and she had a different idea of the role of the masses, which I think you can see most clearly in The Socialization of Society:

We do not need and do not want to dispossess the small farmer and craftsman eking out a living with a small plot of land or workshop. In time they will all come to us voluntarily and will recognise the merits of socialism as against private property.

However, in all the other essential respects, it's quite a good answer and I feel comfortable directing you to it with that caveat. Ultimately, I think it's neither a result of any inherent Russian or "Oriental" impulse to despotism, nor of Marxism or communism as a whole. It ultimately comes down to disagreements over the need for a vanguard party and the role of the working class in the revolution, and when and where which parties on which sides of that question came to power.

EDIT: There was a reply to this comment that cautioned me against exaggerating the differences between Luxemburg and Lenin. It's been deleted, and to be fair, I understand why. These kinds of questions can get very political, very fast, and the reply wasn't really addressing OP's original question so much as it was addressing some errors on my part. However, in the interest of honesty, the reply used this piece by Lenin as a rebuttal to say that Lenin expressed similar ideas to Luxemburg about the role of the masses and of cooperatives in the revolution. My reply would have been the following:

Thanks for replying, and I appreciate the corrections. I don't mean to defer responsibility to my friend — I exaggerated the difference, and you're right to remind me that rhetoric like Luxemburg's speech will always be somewhat more optimistic than actual policy. However, I still don't think it's right to say that the two shared the same views on the masses' role in revolution, or on the value of democratic centralism. If I'm misunderstanding what you're trying to say, and you only mean that on this particular point there was not such a big difference between them, I suppose I have to admit defeat. But I would be cautious generalizing from this single point.

Again, I don't mean to descend into the weeds of the politics surrounding the issue. But in the name of transparency, I want to be clear that I am incredibly far from being an objective, unbiased source myself.