http://www.uesp.net/wiki/Morrowind:Skills http://www.uesp.net/wiki/Oblivion:Skills Overall lack of weapon types, thus less variety. There is no justification on the butchering of the magic system as you controlled the entire system and you could delete spells. People have no self control so they scream overpowered, and that has killed the magic of the magic system, spell creation. The caves was the same thing rehashed over and over, Morrowind dungeons was overall designed better in every way. Oblivion was the series low point in terms of the world. Not everthing in Morrowind wanted to level up with you with thousands of hit points and magicka.
As I said: Weapontypes equals quantity, not complexity. It's good to have, but without small changes to how they work it does not affect game-complexity. And since
none of the weapon or armor skills played differently at all it only takes away from quantity.
There are exactly 11 spell effects in MW that don't exist in OB. 3 of them are not transferable with the new combat system(blind, sanctuary, sound), 3 of them are teleporting(mark, recal, intervention) 4 of them are simple means of transport(levitation, fast swim, slow fall, jump) and 1 that can majorily mess with the NPC AI(lock). The delete spell option was good though. Olivion introdused a few new effects: dominate, reflect damage and absorb magica.
All the dungeons in Morrowind looked and had almost identical layouts, but with more unique loot. You will likely find about equal amount of "odd" caves in OB of you actually takes some time exploring the more remote places. Example of MW reuse of design: The Dunmer "towers" commonly occupied by Telvani or other distrustful wizards: Small opening room with branching options for going to a small, insignificant room or continue further in. A few small rooms scattered about, with a prison cell. Next area: Large, round room with an open area reachable by a dual staircase, but the main path onwards are found by just following the path you arrived on from the last area. Color scheme of dungeontype: Medium biege. I think it is 15-25 caves almost just like this on the island of Vvardenfell.
The leveling in both MW and OB was horrible. MW got to easy fast, even if you ran around with steel armor and a silver longsword after lvl 20 almost nothing stands a chance. OB got to tedious since enemies got too high amounts of health and neither the player or NPCs could achieve very high damage without exploiting the weakness to magica stacking effect.
In short: Neither games are as complex their hard-core fans want it to be.