The dark influences have been a double edged sword.  It is a sword that harry has walked the edge of and on more than one accasion he has overbalanced into the darkness.  His dance on the razor's edge has made him strong.  Strong enough to resist incredible temptations. 

He has drifted into red light saber territory, as detailed in the original post and on other instances.  He has also resisted temptation to grasp power more extensively and more consistently than most if not all mortals.  I do not think the Harry Dresden of Storm Front could have resisted the calls of Laschiel in DB and certainly not .

There is evidence Harry has been groomed as a weapon, and some of this grooming has made him darker and "stained".  It has also forged him like a sword.  A magical sword with a high ego, able to assert his own will and defy anyone who would subvert that will.  Harry's story is the balancing act between the light and the dark.  The fate of the world hangs in the balance.

I'm not so concerned with physical characteristics for Harry, but the acting has to be great.  Harry goes from cynical wise-cracker to towering, righteous rage in an instant.  He can be shy, insensitive, gentle (especially with children), analytical, and wildly emotional.  He has a host of emotional contradictions, this role requires a GREAT actor.  He has to be able to go from smart-ass to bad-ass in the blink of an eye.  Harry has lived a lot of life in his 30 or so years and it should be reflected in his demeanor, not many young actors can portray that.  I think Robert Downey can do all of this.  I would be interested in knowing why you don't think so.

A friend of mine recomended the series to me before the TV series aired.  I watched a few episodes and enjoyed them but still did not crack open abook.  My Wife even bought me White Knight back in '08 but it sat on my shelf for three years.  I read the Codex Alera lat year and loved it.  I found the inclination to read "Ghost Story" two months ago, then overspent my book budget buying the rest of the series and devouring it over a two week period.  I am impatient for "Ghost Story" to be published, but as a Robert Jordan fan I am much pleased at how prolific Jim Butcher is as a writer.  My wife and friends know me better than I know myself.