The capitalist system has changed immeasurably over the last century. The fundamental mechanics of capitalism, that Karl Marx comprehensively analysed, remain within todays economy. And it will continue to be so until capitalism comes to an end. Our society is a class society, and therefore class struggle remains one essential aspect of collective liberation. One conceptual pole of capitalism participates in the ownership and management of the means by which we create the environment and content of our lives and relationships, and therefore has an interest in maintaining the hierarchies (including property relations) of which they are benefactors. The other conceptual pole of capitalism is dispossessed of the above, and therefore has an interest in destroying the hierarchies by which they are subjugated and impoverished. Yet the real manifestation of this abstract class divide is qualitatively different from 1917. I believe I am correct in stating that Marx argued that as capitalism developed the class divide would intensify, as part of society became increasingly 'proletarianised' while the other part accumulated ever more capital. In some ways this has proven accurate: for example the sectors of the economy which lingered from the feudal era were absorbed into capitalism, and for another example we can highlight the concentration of the vast majority of the worlds capital into the hands of perhaps a few thousand people (on this planet of seven billion). In other ways Marx's projection couldn't be further from the truth as the real expressions of ownership and authority in society become ever more graded, and, especially, as relations of power become ever more dynamic and capital circulates ever faster. To put it another way, we can no longer speak of a proletarian class who have nothing to lose but their chains, and a bourgeois class, who own the fields, factories, and offices. Workers have truly bought in to capitalism, we have become invested in the status quo in various ways. The consolidation of imperialism in 'globalised capitalism' makes us all (in the 'first world' countries) beneficiaries of the exploitation of labour. And as far as the bosses go, it is a lot more complex than simple ownership of private property. Control of the workplaces/businesses, control of infrastructure, control of public space, social control, etc. have become the eminent features of the-powers-that-be. There is no categorical divide between a ruling class and a subjected class. In the workplace, for example, there is not a boss and his workers, but a gradation of positions each with increasing authority and pay. The many inter-dependent hierarchies (including but not limited to property relations) which all together constitute our class society manifest in different forms and are distributed across the population in varying and often conflicting patterns. The same person can be advantaged by one power-structure and disadvantaged by another.
This is just one tiny exploration of changes since 1917, as far as I see it; yet it would take a whole library of books to cover everything. I wouldn't hope to summarise all that.
While the reformist left continues to decay (let us celebrate), with the occasional death-throes of social-democracy, the anti-capitalist left becomes ever more relevant, simply because the crises that face our capitalist world continue to intensify and appear terminal. The ecological crisis is perhaps the most pointed among these, because whereas human suffering can be overseen by bullets and antidepressants, the collapse of the ecology which sustains human society and the exhaustion of natural resources which fuel the vital continual growth of markets, cannot be corrected by any process compatible with the continuation of capitalism. Socialism is the primary (perhaps only) viable alternative to capitalism, for those who value freedom and equality.
In my view, the biggest change that the left needs to cope with in order for leftist social movement to remain relevant is the annihilation of authentic human society, that is, authentic individuality combined with authentic social relationships (community), all of which has been inescapably eroded by the community of capital, where all of our relationships are mediated by commodities, where we define ourselves by image, and our grasp of reality is constructed for us as a grand spectacle. Possibilities of liberation open up as we break free from this essential dehumanisation and build new relationships: a process that can only be pursued in antagonism to every fiber of this social order.