Duncan had some successes in his five-and-a-half years at the helm. With 4oD, C4 was the first major broadcaster to launch a fully fledged video-on-demand service. In 2005, it was the first UK operator to simultaneously broadcast on TV and PC.
He also launched digital channels More4 and 4Music, extended E4 onto Freeview and turned Film4 from a niche pay-TV channel into a free-to-air proposition that makes much more money than it did previously. C4 says audience share has gone from 10% in 2004 to 12% in 2009 and that ad share under Duncan's tenure is up from 10% to 25%.
On the downside, last year's 4Digital radio fiasco won't figure in the list of Duncan's achievements, nor will his response to the Shilpa Shetty/Celebrity Big Brother racism row in 2007. Unfortunately, the overriding feeling is that when it came to the "big stuff", Duncan failed to push the ball over the line.
There is still significant doubt about C4's long-term future. Duncan bet the house on securing alternative funding - although C4 denies he ever asked for a share of the TV licence fee - but ultimately failed. His prolonged public pronouncements about
C4's £150m funding gap that had to be filled from external sources positioned the broadcaster as a weakling not in charge of its own destiny.
This year, Duncan failed to secure a partnership deal with BBC Worldwide - the last chance for him to land the big one and leave a positive legacy.
As stated, none of these are easy challenges. But that's why senior TV executives get paid bumper salaries: to address complex and difficult problems and find solutions that benefit their organisations.
One of Duncan's first acts when he joined C4 in 2004 was to scupper a merger with rival broadcaster Five. It would be ironic if one of the by-products of his leaving was the reopening of that possibility in 2010.