Vyvyan Basterd
Adventurer
Where it gets tricky in 3E/4E is deviating from those odds. It is so tempting to adjust the DC by increments of 5 up and down, but that doesn't leave much room for expectations.
So now can write guidelines as a competent character giving a task where results under pressure matters:
DC 9 - slim possiblity of failure, but otherwise routine
DC 12 - easier than normal
DC 15 - normal problem, will success more than fail
DC 18 - harder than normal, could go either way
DC 21 - can pull it off, but risky
4E after Essentials does give a handy guide in a very compact form. It's a handy chart I always keep in front of me. It lists DC by all 30 levels and by three grades of difficulty (Easy, Moderate, Hard). An understandable misconception some have is that the Level in the chart is meant to be the character's level. I don't believe they explain the concept very well, but the table is meant to give you difficulty by challenge level.
They should (or maybe they do for those who've read it more thouroughly, I just took the concept an ran with it) build examples from this table to flesh out the concept. Using the doors as an example, the misconception is that the DC to open a stuck wooden door increases as your character increases in level, resulting in the same relative difficulty as he becomes more competant. Instead, a normal wooden door should be a low-heroic tier challenge and being stuck could be a Easy difficulty. An Epic-tier character should not even need to roll to unstick this door. Building upon that, a jammed door could be Moderate and a locked door could be Hard. And building on the Challenge Level a reinforced door could be upper-Heroic, an iron or stone door Paragon challenges, while specific portals such as the Gates of Heaven or Hell could be Epic challenges while all still falling under the general guidelines of one Difficulty chart mechanic.
What I think is needed is better guidelines for common types of checks made under each skill and attribute with examples by Difficulty and by Tier. As an experienced DM I may have skimmed past such discussions in my readings of the Essentials books, but I do not recall as good of guidance as I would hope to see for newer DMs.