I think it's what Lizard says.
All of these "terrain level" examples we have seen in both the DMG and now DMG II are presented to the DUNGEON MASTER and not what the player sees.
Basically, the only reason there is a door there is because I put that door there (I mean, if I wanted to, I could have easily just have said "the door is open" since if it is anything like my own house, I have multiple doors open at a time) and if I put that door there, I want the PCs to overcome it so I need the DCs FIRST before I need the description.
(You know...I could've sworn the DMG actually talked about this...Found it. Pg 23. Under Realism and no I'm not joking)
"Sometimes realism is a matter of very small details. If two wooden doors appear exactly the same but one requires a DC 16 STRength check to break through and the other one requires a DC 20 check, the world feels arbitary and inconsistent. It's fine for one door to be harder t break down, but your description should give cues about wy one door is so much sturdier than the other, whether it has adamantine reinforcements or a noticeable aura of magic sealing it shut. That makes the game world seem realistic"
It's basically the same method used for the monsters. As a DM, I don't really care "how my monster gets to doing X amounts of Damage". All I care is that at the level at which the monster interacts with the PCs, it does that much damage.
I don't care about feats and all of that and I think that's basically the 4e DM method. Don't sit there and try to figure out "ok, should I have a wooden door - no the DC too low, a adamantine door, no the DC is too high" but rather "ok, this is the DC for the door - hmm, this is inbetween wood and adamantine, ok, I have a description"