If you look at one of the early pets, such as Mr. Wiggles the pig, his special motion was pretty unique: he ate grass while idling.
We were all "Oooh, look at that grass eating!!"
Now consider the Perky Pug and Core Hound Pup. Not only do they have special idle animations (the pug wipes -- yes, there's a pun in there -- and the core hound rolls over and tosses a bone from one mouth to the other), but they also move. They don't just stand there with a twisting tail and occasional grass-eating. They shift from paw to paw, they look around, they play while they wait.
(For the record, considering the number of people getting hacked these days, I approve of only giving Core Hound Pups to people with authenticators. I thought "I wonder if this authenticator will be a hassle" when I got one and it isn't. At all. It doesn't add more than 2 seconds to logging in unless you have outside forces at work, such as a sucky connection or an unreal tendency to lose things or dyslexia, and account restorations are draining a lot of Blizz resources that could be used to make us more pets.)
What I'm interested in seeing on the pet front is whether or not Blizz continues to give us more of these evolved pets or whether these pets are special "Oh we just had some spare time between people getting hacked" items.
Personally, I don't mind getting fewer pets in the future if they all get this level of time and detail. I don't know anyone who would say "Yes, give me lots more generic recolor pets instead of a couple of really cool new ones."
Crazy people, maybe.