The hardest part by far is getting good motion out of the characters. To start with there's the rigging issues. Bad deformations are the first stumbling block, but otherwise, without a good and solid rig there's no way to get decent animation. But even with that, getting something that moves convincingly, reads well, and exudes the character's "character" that's a real bitch difficult thing. Rigging, modelling, lighting, all just technical issues that anyone of a sufficient intelligence can grasp with time. Animation, now that's an art.