In older editions, the H2 Ghost was awesome. Scary, hollow. Everything about it was dreadful, the sound when it flew towards a peasant stack (just after multiplying itself when killing those weak bastards, damn it!), the way it screamed when being hit. In 3, wight and wraiths were a reasonable ok. Then, they completely and utterly screwed up in H4. H5 was less terrible, but still, gleaming all the while.
Liches were so nice in 2 (rather cute in 3), with their ranged attack that seemed truly devastating. In 3, it seemed that the attack lost its impact, became more like a poisonous gas only. And their art was also hollow. The crown gave them an aspect of lost grandeur.
Skeletal dragons and Ghost dragons in 2 and 3 were also pretty neat.
Even zombies in 2 and 3 had some undead feeling about them that's nonexistent in H5-6.
Regular troops, or anything except for Liches, should be lifeless, like this picture:
The most alive feature applicable on undead, imho, would be a faint gleam in the depths of big black eyes, something along these lines. A thing that you would look upon, see that it is dead, but still, there's something wrong about it, something is there that shouldn't be. As I recall reading somewhere: "When you look too much into darkness, darkness looks back at you".
