Grievous had the potential to be a great character, but the way he was portrayed in Ep.III was terrible. The first thing 90% of people saw of him was walking onto the bridge of his ship, coughing, and playing Three Stooges with his battle droids. If you watched the animated Clone Wars shorts, the last thing they showed was Grievous capturing Palpatine and one of the Jedi trying to force crush his body, hence the coughs(I may have some details off, it's been a long time). In those cartoons, the Jedi were scared of him because he strait up murdered a ton of them. But not a lot of people saw those shorts.