Though Daggerfall had a world of sorts with towns and wilderness, it wasn't nearly as detailed and immersive as its successor, Morrowind. Morrowind was a game with a fixed 3D landscape, cities painstakingly hand-built by the developers and with some real personality. Oblivion expanded on that later on by making the cities larger and add more realism to them. However, with the larger, more crowded cities of Oblivion also came the issue of walls. In order to save your computer from blowing its cycles on crap it didn't need, cities were placed in cells that required loading.
Another example was voice acting. Morrowind didn't have much voices. Oblivion improved on this by giving every single NPC out there a voice. But this backfired in a different way. The voices quickly became stale - after all, voices were shared between races and thus NPCs. And there were so many of those!
The cities and the voice acting are just two of the many examples, but we can definitely see how improving something on Morrowind had backfired in another way. Todd Howard's team, a group of fantastic programmers with some amazing RPG titles firmly under their belt, will have learned from this. This will be their third modern open world game, and they have now experienced that there's no such thing as a free lunch when it comes to making games like these.
How do you think Bethesda will use this newfound wisdom? What have they spent their time on? For example, I recall them mentioning they revamped their engine to be a bit more modular, so they're able to take out one part, modify it and plug it back in without causing a total collapse of the rest of the game.
What do you think they have learned from Morrowind and Oblivion?