So for me the best/most enjoyable areas in Dark Souls were: blight town, the catacombs, the painted world and the tomb of the giants.
The first 2 were favorites for several reasons. The multiple levels meant you could find some shortcuts or secret hidden areas with really useful loot (Eagle Shield, Gwyndolin Ring, Secret Blacksmith, and even a pretty hard npc summon considering Leeroy). I think the areas also had some of the hardest enemies- bonewheel skeletons, blowdart guys ect. I loved those two areas on my first playthrough as difficult as they were and I felt they had the best replay value as there was always more to find.
Again the Painted World was hard to get to/discover from the start but once you got there it was just magical. So much different parts to jump/explore with a massive aray of enemies and items to find. The path through the level became a bit hard to understand in the second half and just made the player more confused but also forced them to explore every inch of the area which was awesome.
I liked Tomb of the Giants because it really was hell. Requiring a lightsource is instantly off putting and the enemies could crush you quite easily. Again it had some really hard to stuff to find/retrieve (serpent ring, divine ember) and then the skeleton babies at the end were the perfect touch.
Don't get me wrong every area in Dks had it's own secrets and places to explore but these were my personal favorites on and Ashe Lake for the atmosphere.
In Dark Souls 2 I want there to be multiple routes to the boss fight (which they might be doing because I heard you can fight bosses earlier if you want to?). Multiple levels with short cuts and hidden areas that even if you find will probably just kill you anyway. Sens Fortress and Anor Londo had some secrets but as levels they were pretty linear and there was always one clear path as was Lost Izalith although I heard that was unfinished?. I felt aside from Ash Lake, Dark Souls was lacking in atmosphere compared to Demons Souls but maybe that was just me?
So what do people want the areas to be like in Dark Souls 2?