1. I want dungeons to not to be so cramped up. It shouldn't look like this: http://s2.n4g.com/media/11/newssi/15000/18861_0_org.jpg
Add a greater distance between dungeons. It's not realistic or fun that there is a huge dungeon every 100 feet.
I also want dungeons to be in more strategic places, rather than having a big underground dungeon in the middle of a forest.
2. I want dungeons to be more unique. The dungeons in Oblivion all pretty much felt the same. Eventually when I was exploring and saw a dungeon, I thought "Bah. It's just like all the other dungeons. Nothing special. No need to explore."
3. Kind of off-topic, but I don't want any "magic icons" that shows you the exact locations of all nearby dungeons and other locations. It completely removes the excitement of exploring and finding supposedly hidden dungeons (which really didn't exist anyway in Oblivion), according to me.
All of the above points pretty much ruined my incitement to explore dungeons in Oblivion. A big thumbs down in a roleplaying game.
I'd say 65% quality, and 35% quantity. That's how Bethesda should focus
In Oblivion it felt like 80% quantity and 20% quality.
Having 300+ dungeons, of which many are the same like in Oblivion, is really bad in my honest opinion. Because it will end up, in theory and in feeling, you exploring only a few different dungeons.
I'd say variety is the key. That meaning both how the dungeons look & feel, and how they are placed in the world. I think having 5 dungeons within 100 square feet is bad.