Morrowind was rich with atmosphere and experiences. There were more factions. Less leveling of every item and creature. More varied terrain and landscape and architectural styles. More political intrigue, from quarreling Great Houses to an uneasy occupation by Imperial armies to the ever-baneful presence of the cordoned-off Red Mountain, a strained population visibly struggling to get by while dealing with increased social strife from immigration and poverty and a corrupt monarchy led by a god-king that hadn't been seen in years.
Oblivion was bland and stale by comparison. Its landscape was basically a copy-paste of random trees in a lush green landscape, varied only with a bit of snow in the north and a bit more water in the south. Every city was standard and identical, made up of essentially the same stone walls and wooden buildings and stained glass windows. There was no great social strife; the death of the Emperor, while an interesting backdrop, didn't seem to have any actual effect on the goings-on of the rest of the Empire, no Counts or Lords or fractions of the imperial council vying for control. Your choice of dungeon consisted of caves, white ruins and grey ruins. Your choice of profession limited to Warrior, Mage, Thief or Assassin (in terms of what factions you could join).
It was a real let-down, a boring over-reaching game that failed to meet up to the bar set by Morrowind and by all the talk of "dynamic NPC interaction" promised.
So has Bethesda learned their lesson? Will Skyrim be another bland, repetitive copy-paste job? Or will it be an interesting, dynamic world for us to play our roles in?
I'm obviously hoping it helps rescue the floundering TES line and turns out to be another smashing success like Morrowind.