However something people have to keep in mind, there is a big difference between "complex" and "complicated". For example, in my opinion at least, the inventory management in Oblivion was not complex, it was simply complicated, especially when comparing it to Morrowinds "All on screen" Inventory.
Just saying "everything needs to be more/less complex" doesn't work, the problems need to be tackled individually. The main decision point here is "what makes it more immersive, what makes it more user friendly".
A few other examples (those are my personal opinions, feel free to add your own):
-The map:
I'd like to see the return of a mini map as having to constantly go to your inventory to check it is annoying. Some suggested a method similar to FarCry2 where you push a button to bring out the map and look at it real time. While this sounds interesting I think it's still a break from the games flow.
Of course however both can be in the game the same time, you can have a mini map and, when you need a bigger one, just press the map button to display a larger version.
-Health/Injure system:
This is one where the game should take a step up in complexity. Just having a hitpoint system can be very bothersome as Oblivion showed, it pretty much boiled down to hack and slash (and no it was not due to how the combat system worked, it was the hitpoints and level scaling system).
A lot could already be done by a locational hit system but even more by a injure system, even deeper with one that takes the kind of damage into account.
There are some good examples of games that have very detailed injure systems and still work well like "Call of Cthulhu: Dark corners of the earth" (by Bethesda BTW, take a hint from yourself guys), "Deus Ex" and "Dwarf Fortress".
I once posted a system that had a huge list of different hit locations and their effects, many said "that's too complicated", however what it would have done is giving weapons realistic chances of damage rather than basing it purely on a damage value, like an arrow being dangerous because it can pierce vital organs and not because it has a high damage value.
It wouldn't require more by the player really as it would happen automatically, it would simply expand the healing system and how you can strategically fight opponents.
I can actually see the only bigger problem be that it removes the "tough guy" stand point who can take a million hits and still stand and replaces it with "you can be as tough as you want, if you get hit you die like anyone else". And that many think it would mean "arrow to the head = dead" but it doesn't have to, it would be severe but doesn't have to mean instantly lethal.
-Alchemy:
Here it could use a up in complexity too, not necessarily to make it more complex to use but simply to get more out of it. Why weren't you able to make your own bone meal when you could find bones everywhere? Or grind wheat to flour? Why was a pumpkin that weighs 5 pounds the same as a single strawberry?
Here it should take more into account than just how much you put into it, there could be actual potion recipes where you still put things together by effect but there are exact measurements which gives the best result.
Well these are just 3 examples of what could use more complexity and what could be simplified, it's still a matter of what could be considered complicated rather than complex.
Some systems could be both considered to become less and more complex within certain fields, the Leveling for example. Here new mechanics could be added that both expand the leveling system http://www.gamesas.com/bgsforums/index.php?s=&showtopic=995978&view=findpost&p=14421066 but the same time simplifies the mechanics to be less number crunching by adding automatic and direct leveling and distribution.
But what are your thoughts on what should be more complex, what should be simplified and what was fine?
Please, as mentioned before, don't just say "It should be more/less complex" but actually give examples.