My Difficulty Class by Tier Rules

I key DCs off the Tier of the encounter/environment, which is often the same as PC level, but not always.  

My DCs:

10: Easy Heroic task
15: Moderate Heroic task
20: Hard Heroic Task, Easy Paragon Task
25: Moderate Paragon Task
30: Hard Paragon Task, Easy Epic Task
35: Moderate Epic Task
40: Hard Epic Task, Easy Supra-Epic Task
45: Moderate Supra-Epic Task
50: Hard Supra-Epic Task

"Very Hard" = Hard +5.  "Impossible" = Hard +10.

"Supra-Epic" is for environments and threats created by creatures over 30th level, eg Orcus.  A task that was 'impossible for Orcus' or similar would typically be DC 60.  Tasks impossible even for greater powers, such as breaking the chain that binds the Fenris Wolf, could be DC 70 or higher.

The DCs are roughly typical for PCs in the mid-point of the Tier, eg Level 5 Heroic PCs, Level 15 Paragon, Level 25 Epic.  Towards the top of the Tier PCs will begin to encounter task DCs from the next highest tier, just as they begin to encounter monsters from that tier.  So eg high Heroic Tier PCs adventuring in the dungeon of a mad Paragon wizard will be dealing with Paragon tier DCs.  If the wizard is low-Paragon then most of the DCs will be Easy (20) or Moderate (25) for Paragon.

The great beauty of this table & this tier-based approach is that I can keep it in my head, I never need to look up the table in-play.