Ghosts were a joke (except in that one quest with that dude and his house in anvil, I think they were placed well in there, and it was interesting to see that quest unfold, albeit rather lamely - but hey, who's criticizing the poor bloke for not wanting to hang around a bunch of poltergeist?). Wraiths actually posed some threat with their faster striking/ability to wield blades, but were still easily handled as long as you stayed quick on your toes.
Zombies... all right, with the recent uproar in Zombie games I think Bethesda should REALLY strive to make zombies a little more threatening this time around. You see one - and only one - in the catacombs as you're fleeing with the Emporer. Wouldn't you see - if you saw one zombie in creepy tunnels - many zombies around? Dread Zombies were a little more fascinating with their pestilence like haze, and stronger strikes, but their movements were completely predictable like every other zombie. PLUS! They ALL looked the same, give some had missing limbs and/or heads. I'm guessing they were all either imperials or bretons. I guess I can see why they did this - or could possibly justify this - because most dead families buried in Cyrodiil would be imperials, so... whatever BGS.
Skeletons posed the largest threat, imo, solely because you could come across several at a time instead of just one, and at higher levels they all had the ability to summon a buddy bag-n-bones to the thrash party dedicated to your soon-to-be ravaged corpse. There were two military types of skeletons: bow weilders and blade/blunt weilders (of course there are the armored variant when amongst the ranks of the dark brotherhood, but you only see those through that faction's quest line - and even then they were exactly the same except with a kevlar type vest on which I'll never understand where they found kevlar, and I wasn't even able to loot the body for it!).
And last but not least - LICHS! Bunch of hovering pansies. Their pathetic ability to summon a "lesser" undead and zap you with whatever magical ability is charged up in their staff still didn't even make up for their supposed "powers". Aren't Lichs supposed to be powerful sorcerers who have ravaged their own body and become immortal (in the long life sense, not the I'll-smack-your-face-with-my-sword-50-times-and-you're-dead kind because obviously we were able to do that and they fall over limp just like any another creature in the game.) They're not even impervious to regular weapons whatsoever! At least, regular weapons should do half as much damage to them or something. Of course, a magical weapon kills them "faster", but hey at least you can kill them with a well placed iron dagger in their spine with 6x sneak! (<--- exaggeration, don't take this part literally)
So, give me your thoughts, O' ye ears of Nirn!