When I first saw both trilogies, never was I left with the impression I needed to know more. All the cartoons, extra books and so on is just new stories, you either like it or not, there isn't a single need to know thing in the cartoons that aren't in the movies (SW wise).
This was true of the original, not of the Prequels.
Many things are left to other sources if you actually want to understand what was going on in the Prequels. We had a huge discussion about this, but here's the most important question, what the hell did the Sith want and why?
True, they are just evil sons of bitches. But why do they hate the Jedi. 6 movies and we don't have an answer in any of them.
Plus you have supposedly important characters like Grievous who have no background or story.