Is this so different from 4E, though, ultimately? One could parallel HP ablation in combat to the staged progress of SCs. How does a "success" in combat check (successful combination of d20 roll plus modifiers, inflicting damage or status effect; now it's the enemy's turn to strike back, i.e., GM introduces a complication in the form of HP ablation, status effect, etc.), all as stages toward determining the outcome of the combat, differ from "success" in a SC check (successful combination of d20 roll plus modifiers; GM introduces a new complication)? In short, aren't the mechanics of combats vs. skill challenges basically symmetrical (some number of d20 rolls plus modifiers until a target number of total successes is reached)?
While in theory I like the tightness of the d6 (or 2d6) mechanic as compared with the d20, both systems seem to me to share a rough symmetry in how mechanics work for both combat and noncombat resolution, particularly as DCs in SCs scale to fit the changing fiction, just as Defense scores, HPs, damage, etc. scale with the same.