There are lots of other problems too. 4e doesn't have skill points or other resources to allow characters to emphasize a particular skilll.
Feats.
Your 8th-level rogue is probably about as good at thievery as any other 8th-level rogue
The truth is that two 8th level rogues
should be about the same ability for a core skill. Having a core skill being wildly at variance with other similar characters is a flaw. A difference of 3 points is significant in the D&D system.
We find some interesting things when we examine the DCs on page 42:
The Easy check needs a 5+ on the die to succeed, assuming the PC is untrained and has a +0 bonus in the ability score.
The Moderate check needs a 5+ on the die to succeed, assuming the PC is trained in the skill and has a +0 bonus in the ability score... and continues to increase the ability score with every ability score increase (4th, 8th, 11th, 14th, etc.)
The Hard check needs a 6+ on the die to succeed, assuming the PC is trained in the skill and has a +4 bonus in the ability score, and continues to increase the ability score with every ability score increase (4th, 8th, 11th, 14th, etc.) For your unskilled lout, Hard means 15+ to begin with, increasing to 18 by the end!
I have a suspicion that for lone skill checks, you might want a +5 to DC; in skill challenges, you need easily accomplished rolls because otherwise they can be very hard if you're using the wrong skills.
But that brings up one of the chief problems with Skill Challenges: as written, not everyone needs to participate, and it's too easy to just use the right skill again... and again... and again...
One PC uses Diplomacy 12 times for the win? Yeah. Too easy.
The idea of Skill Challenges is great; the structure isn't there yet.
Cheers!