Scientific issues aside, Walking With Dinosaurs was an immense success because it drew viewers into the lives of prehistoric creatures. They were living, breathing creatures, and audiences couldn't help but care about their fates.
Planet Dinosaur has two things going for it: solid science, and a great actor doing voice-over. In all other ways, it is greatly inferior to a series made a decade and a half before.
It is ugly, which for a big, expensive "spectacle" show is unforgivable. Every visual aspect is terrible: WWD's lush real-world locations have been replaced with flat, bland CGI backgrounds that would look disappointing in a video game; the dinosaur models are beautiful, but they are stiffly animated which makes them feel completely devoid of life; and the entire finished product is just terribly rendered. This is abysmal CGI, and the BBC bragging about how it only cost one third of WWD's budget is not a selling point; it's an explanation for why it's so damned ugly.
It would have been so easy for the BBC to hire Impossible Pictures and the whole WWD crew and make a sequel series that kept the heart and soul of what made WWD great while polishing up its more problematic aspects. Instead we get Planet Dinosaur, a series so ugly to look at it makes it seem like it was made BEFORE Walking With Dinosaurs, not more than a decade after.
I was so thrilled that the BBC had made another dinosaur series, but the finished result is deeply disappointing. I am one seriously unhappy dinosaur nerd.