WaveNet is probably modeling the source data very well. It sounds like they just need more data with emotion and inflection, rather than having source data that is optimized for monotonicity and precision.
If you work in radio or voiceover you learn really quickly that the voice is so much more complex than people give it credit for. Subtle changes in delivery, timing, inflection, syllabic emphasis, pauses etc... make a massive difference.
Anyone can "talk like a robot" but speaking naturally is way more dynamical than just making the sounds of the words transition smoothly.
I'm not sure if it's way easier or way harder than we're doing it now.
EDIT: A big example of that is the Japanese example at the bottom. English has had far more effort put into the old model, so it's already pretty good. But the difference between the old and new Japanese voice is really striking, and they were most likely able to make the new one much faster.
I wonder what compromises they made to improve the performance by 1,000x. There have to be some.