Not every DC varied with level. In fact, this was part of the assumed design -- not every skill was WORTH one point per level, so if you only invested a handful of points in them, you could get as much functionality as you ever would really need.
But anyway, this is kind of a fiddly bit of specific detail. The main point is that 4e lost a significant amount of granularity that made noncombat challenges less fun to run, rather than making them MORE fun to run.
Well, fun is subjective of course. I found that if a skill wasn't scaling per level then I was simply asking for a roll that didn't need to done:
DM: "Ok, make a knowledge(arcana) check to determine what the runes on the wall mean. DC 10."
Rogue: "I have +5 to that. I roll a 5. Whew, I made it!"
Wizard: "I have +27 to that roll. What is it?"
Rogue: "Why did I put ranks in that again?"
DM: "Fine then, you know what the runes are. But make me a search check to search the room. DC 20, it's pretty hard."
Wizard: "Ooh, I have a 28 Int and I'm an Elf. I get +11. I have no ranks in it. I make it."
Ranger: "That sucks, I put 10 ranks into that so I could be good at it. But I have Int of 8, I only have +9."
Rogue: "It seriously doesn't matter guys. I have +25 in Search."
When making a skill check in either 4e or 3e, it doesn't matter what your skill check is if it isn't within 10 of the highest person in the group with that skill unless it was one of the couple skills where individual successes matter(Jump, Climb, Swim, Spot, Sneak, Hide, and maybe a couple of more I'm not thinking of). For most skills, however, it is simply a matter of whether SOMEONE in the group succeeds, not how many or who.
As for the amount of fun to RUN these challenges. My experience is that they are exactly the same:
3e:
DM: "Give me a search check."
Player 1: "I got 17"
Player 2: "I got 20"
Player 3: "I only got 5"
DM: "It was DC 20, you find a blood smear on the floor underneath the table."
4e:
DM: "Give me a Perception check."
Player 1: "I got 17"
Player 2: "I got 20"
Player 3: "I only got 5"
DM: "It was DC 20, you find a blood smear on the floor underneath the table."