He was classy, effective, actually likeable, reasonable to an extent, and yet still glaringly cruel and very morally ambiguous.
I'd be hard pressed to recall for any moment found in TCW that actually made me feel interested in his ideals and motivations, since we only ever get to see him either being ridiculized, battling againts Anakin and/or Kenobi, or acting obscenely evil. You don't write villains by beating the audience into hating them any chance you get.
That's the thing: CW 2003 treated the character with respect, even if it didn't leave much room for long term development (you have entire novels and comics from the same continuity dedicated to that anyway). In TCW he feels like another nuisance the writers had to take care of but weren't exactly invested in, because when Asajj Ventress and Darth "Ricardo López" Maul feel closer to the audience than the two main antagonists of your Clone Wars setting, then there's something very, very wrong with the writing.