The defining feature of capitalism (for a Marxist, anyway) is the private ownership, control, and accumulation of capital goods. In practice this tends to mean that productive capacity becomes centralized, and as a consequence most people exercise no control over what we do with accumulated capital as a society: they merely sell their labor / time for a wage. Also since productive forces are more centralized, productive relations tend to become more socialized as well (the difference between working the land as a peasant in an economy with low development, vs working in a factory as a laborer in an economy with higher development).

Note that the centralization and socialization of production is regarded as a good thing here. You get greater economies of scale and thus greater productive capacity when this happens. This is why Marxists view capitalism as a progressive historical development - we just don't fixate on it as though it's the pinnacle of human productive relations or the "final step" or something like that.

The main problem with capitalism comes when you have a great degree of centralization and control at which point the profit motive inherent in capitalism tends to produce a ton of rent-seeking behavior which drags the economy down and makes further progress difficult or impossible. I would argue that in most industries we reached this point decades ago, or even a century ago in some cases. Tech is a notable exception, though we're reaching the point there as well.

This is the point at which reformists will say "well you just need to break up monopolies" but it's like I said: social, cultural, and government institutions tend to bend toward the economic paradigm more than the other way around. The economic base tends to influence the societal superstructure more than the other way around. This is mostly because, day-to-day, people are more focused on their material needs than they are on ideology and culture. So, our government is much more reluctant than it should be to break up monopolies, our media is dominated by capital interests (to control public opinion in favor of capital interests) and so on. Idealists will insist we're "not doing capitalism right" or something but that argument is not persuasive if the long-term result of doing capitalism "right" is that you eventually stop doing it "right" as government and culture bends toward capital interests - which seems inevitable.

And, anyway, remember that the centralization of productive capacity and control is a good thing. It is more efficient, at least until the rent-seeking behavior starts to assert itself and drags the economy down. So breaking up monopolies is kind of a regressive course of action. It would be nice if we could have highly-developed productive forces with a great degree of specialization and centralization, etc., but without the rent-seeking behavior. In order for that to happen you need to keep the accumulation of capital but get rid of the profit motive - or at least remove it as the primary driver of economic activity. Marxists identify the working class as the only class that has the correct incentives to make this happen: capitalists won't do it because they won't benefit from it personally and in fact stand to lose out personally if it happens.

And then, from there you have 150+ years of history of trying to figure out the best way to do that, usually in the face of violent reprisals from capital interests. A history filled with success and failure and lessons learned. A history I'm not going to get into here because I've already typed a massive wall of text and need to get back to work - I'll just say that IMO China has done a lot of good work in this regard but they still have a long way to go.