I think its the characters that make the cinematics come to life. yes, as you say, Arthas was too awesome!
The same stands for both Illidan, and then Deathwing when he stomps on stormwind as they were such epic characters. I giant mad dragon bent on world destruction is a good baseline for any character...
Some of the most memorable cinematics feature the best blizz characters. Firelands + Ragnaros for example. even going back to W3 when thrall and hellscream battle mannoroth! all great characters.
You could say that arthas/illidan as characters has been around for ages and have had years of character building, which would be true, but i feel that that is why their cutscenes have so much more of an effect when you see them.
I do agree with the original poster though based on the reason that, frankly, the panda characters were kinda weak and uninteresting (to me maybe) and not very well "fleshed out". There was little in the way of building momentum. Thunder king for EG, appeared one patch, dead 2 months later. kinda meh... could of made more of him.
My hope is that, as blizz are "going back in time" (yeah i don't like that as a rule) we can see more of this character building, and see cutscenes featuring awesome characters once again.