Streching it a bit there Sil. I mean he's just just the bad dude. Not at all. I walked out of the theatre feeling like I'd watched half a movie because all of the shit needed to bridge 2 and 3 happened in EU. Everyone's going on about this bad dude who does nothing terribly bad, or badass, the whole film. He's a pretty major part of the third movie, but if you haven't seen any Clone Wars stuff you don't have any idea of who he is or what he's capable of. You end up feeling like you've watched the last two acts of a story and missed a pretty major act 1.
We're meant to infer that Grievous and Dooku led the assault on the Republic capitol and the Chancellor's (ostensible) abduction, which seems like enough setup to me. He also spends most of the movie as the only visible leader of the droid army, so is the focus for ending the war that we're immersed in from the first shot of the film.
Personally when watching RotS, while we do get dropped into the middle of an ongoing situation (not unusual in a Star Wars film), I don't feel like there's any struggle to pick up the threads - and that's having never watched a full episode of Clone Wars. The intro crawl says "there are heroes on both sides", that Grievous is the droid leader, and lays out the current picture. That's all we need, surely?
Might be a bit different if he was the film's big bad. Darth Maul and Dooku probably needed their capabilities more deeply ingrained before the third acts of Phantom Menace and Attack of the Clones. But although we do eventually find out that he's a serial Jedi killer, Grievous is generally more of a political/narrative pawn than a personal threat.