Marx vs. Althusser

On the surface, it seems as though Marx's ideas on society and ideology differ from Althusser's in that Marx looks at the issues from the perspective of production and all that it entails while Althusser on the other hand, takes on the perspective of reproduction instead.
When we begin to look further, however, we see that this subtle difference does not even begin to encompass the complex and expansive reasoning of Althusser's critique of Marx.  (Though to be fair, critique may not even be the correct term as Althusser is simply adding on to Marx's ideas, not entirely refuting them).
Marx discusses ideology as an idea that is completely abstract, up in the air, and one that has distanced itself entirely from anything rooted in what is happening in the real day to day lives of the people Marx is surrounded by.  While Althusser doesn't disagree that ideology, in and of itself, is all about abstractions and complexities that go beyond the material, he also doesn't agree that certain ideologies are, in fact, woven into the material society.  Althusser brings a rooted, real life version of ideology into the everyday by recognizing it in real world institutions "which present themselves to the immediate observer" through, what he calls, ISAs or Ideological State Apparatuses.  These include, but are not limited to, the church, the family and culture of society.  Bringing it down to Earth even more so, rather than simply describing these ISAs, he explains them by instead relating them to what he calls RSAs, or Repressive State Apparatuses, things like police force, the Army, Government institutions, etc. 
Though at first it may seem like Althusser is creating an opposite dynamic by juxtaposing these two types of state apparatuses, he is clear not to hold them completely separate, as Marx so often does.  Althusser recognizes that the Repressive State Apparatus could not operate without ideology, just as the Ideological State Apparatus could not operate without some sort of installment (usually expressed through repression by part of the institutions that teach it).  "There is no such thing as a purely repressive state apparatus" he says, they repress by violence true, but only secondary to the values they hold to make these actions justified.  Values, in this scenario, become ideological ones, ones that are taught by ideological institutions such as the school, church, and family systems.
Ultimately, the difference in Marx and Althusser's representation of ideology is this: Althusser wants us to understand ideology, not only for what it is, but what it does and in what ways it may be affecting our day to day lives.  Marx wants us only to disregard ideology as a whole, without going too far into detail about what it really is in terms of an objective definition of it.
Maybe this is why Althusser felt the need to compensate for Marx's lacking theory of ideology and "fill in the gaps."