The Glass Half Full - Are there ANY positive aspects of Marx?

I am currently studying Karl Marx and his henchman (or boss, as can be argued) Engels in philosophy class. Needless to say he is presented to the class as a God. I don't think anyone in the entire class except for myself has raised any objections to historicism as a bogus methodology. But who am I to criticize people, if they want to believe in unicorns and hobgoblins, I have no right to stop them. Nonetheless I wanted to write a little piece here about the positive aspects of Marxism (yes, there are some!). Every man gets equal treatment on this blog, but this might surprise some people seeing as so far the only epithets I've applied to Marx have been "liar" and "evil" (or a combination thereof). What I would argue, however, is that Marx reaches more or less the right conclusions despite his erroneous base (historicism). He describes the progression of human civilization rather decently as well.
I do like the way Marx sees the state as jut another instrument of exploitation and a cause of economic inefficiency (although in many ways I do not believe this was genuine on his part as in the short term he argued for tyranny of the proletariat in the form of a state). He also wrote that the ruling classes exploited other groups - which is true. This can be especially seen in his early societal stages such as slavery and feudalism where I think it is pretty clear to everyone that exploitation is going on. The slavemaster exploits the slave and the feudal lord exploits the serf who homesteaded his own land (this is different from allodial feudalism which I have written in praise of). I also believe that such situations are prone to create a revolutionary class, as Marx writes. After all, once the exploited realize what is happening and are able to unite in sufficient force, they will rebel against their oppressors. Thus the slaves rebel against the masters and the serfs and burghers rebel against the feudal nobles. It is only natural that at some stage the citizens of the state will rebel against its machinery and ruling elite (this is in the future, of course).I do actually also follow Marx's reasoning when it comes to "communism" being a final stage of development in humanity. In this Marx is consistent, I believe, with Herbert Spencer and Jesus Christ. His description of communism, however, is rather faulty. Not only that, it also involves "unnatural man" - Marx himself writes that communism will be achieved by a new "socialist man".
The problem here is that Marx takes his analysis too far. Instead of sticking to actual descriptive fact (which he is not a master of, but is at least decent at) he continues delving deeper into some strange future scenario where common modern rules of reason are not followed. He denounces capitalism as exploitative despite the fact there is clear evidence that it is not - both parties to a capitalist trade benefit ex ante, not just the "exploiter". Thus there is no reason to think that in a pure free market organized society exploitation exists at all.
In any case I still believe that because of Marx's utter lack of epistemological consistency and covert evil motives the term "Marxist" can still be used as a pejorative by us libertarians. Nonetheless, please everyone, don't spill the baby out with the bathwater!