Uh... OP, this statement is not true at all. Morrowind is a mix of 90% normal swamp villages, towns, and about 10% different with mushroom towers. Hardly anything to wow a gamer now or even back then. Thats not Morrowind's strength, and Skyrim unique terrain puts it to shame if even from just some dungeons alone. This is a case where someone has fond memories of a game and somehow the good things about it get multiplied beyond what it really was.
Its been said, but since it was directed at me I feel compelled to reply. Here are some numbers for you. Take every traditional imperial style castle, fort, house, etc and combine them into one city. That city would still be smaller than the floating pyramids of Vivec. And Skyrim's unique terrains? Mountains, river, mountains, valley, mountains, snow, mountains... oh and some steam vents and glaciers. Morrowind had mountains, it also had valleys, and rivers, etc. It also had a volcano, lava rivers, ashlands, steam vents, islands(not icebergs, real islands with towns on them), pyramids floating on water, a rock you could go inside floating above said pyramids, living mage towers, deadric ruins, and if you combine Solstiem is has snow, glaciers, and more mountains. Yes Skyrim's terrain is pretty thanks to better graphics, but it is no more unique than Solstiem with more even more dull cyrodil still imperial buildings.
The only semi unique city in Skyrim is Markarth, and Morrowind had plenty of dwemer ruins.
And Whiterun is really original. [img]http://upload.wikimedia.org/wikipedia/en/2/2e/Edoras.jpg[/img]