1. Arbitrary way of describing the difficulty of skills...a wall is DC 30 because it's slippery and slick, a lock is DC 30 because it's listed as paragon-tier...
It's not arbitrary. Rodney explains the intent well, and I think its quite clear. In the DMG you are given the tools to handle any mechanical aspect of world building and adventure design. You are shown how to assign appropriate DCs to tasks. The PHB, under skills, has a few DC examples based on common adventuring tasks that were created from the guidelines to assigning DCs (or maybe a bit of chicken and egg, either way). A DC 30 wall is paragon tier. Between the two, you have the understanding that DCs should be based on level and examples of what that looks like in a couple of cases. You know from the DMG that a paragon tier wall should be around a DC 30 and from the PHB that a DC 30 wall is going to be very tough to climb, it will be smooth and slick. The two aren't contrary to each other, the DCs in the skill section are illustrative. A paragon tier lock would match the difficulty of that paragon tier wall. What does that mean?
Well, unless you or your group are experts on locks, it doesn't require a lot of specific description.
Paragon lock:
Paragon Rogue: I examine the lock
DM: It's quite complex. You think you could get through it, but it may take awhile.
Low epic lock:
DM: You've never seen a lock this extraordinary. You seriously doubt you have the skill to get past it, but you won't know unless you try. And with the right tools and a bit of luck, who knows...
And of course, story matters. If the PCs are encountering the lock, it is often because they are meant to go through it (I said often, not always). You may through an epic lock at a paragon party because you don't want the PCs to just stroll through it like a standard obstacle. In which case, you have probably seeded other areas of the adventure or situation with some aid, - perhaps a skill challenge to get some information on how to get past a layer or two of the lock (like the vault security layers in the first Die Hard movie), or a set of tools that can be acquired to give a bonus, or maybe just a collective effort on the party with the bard increasing competence, the aid of a magic item or a potion that boosts dex based checks for a moment, things to take it out of the ordinary.
Also, Imaro, it is not a fact that these things are not clearly explained in the books. Comprehension is as much a part of communication as writing. A lot of people had no trouble understanding it, so it was not "clearly" badly presented. Some things maybe could have been presented better (skill challenges I'm looking at you), but many 4e players seem to have gotten the message.
I really do think a significant source of the disconnect is going from an edition that supported strict RAW, and the legion of players who whole-heartedly embraced that have difficulty wrestling with an edition that rejects strict RAW in favor of a more DM and story interpretive approach. "The DC is whatever you need it to be, now here's some guidelines" is anathema to certain types, but its freeing and liberating to many others and easy enough to follow for a lot of casual players who don't think about these things nearly as much as those of us spending hours online discussing these things do.