I just want to apologize in advance if what I think has been said over and over again, but I didn't have time to read all 95 previous responses.
I am in my early 30s and have played since I was 12 or 13, but more seriously in college. I'm 6'5", big and athletic frame (not skinny like Keegan or Dustin Johnson). When I got a custom fitting in college I was fitted at 1.5" inches over standard. I have used those clubs while until this year (I was away from the game for about 4 years while living out of the country). When I started playing again, I chopped the top have inch off in the beginning of the year and it made a huge difference in my ball striking consistency (not over night, but over the first half of the season when I got my accustomed to them). I bought a new cheap set of irons to experiment with played those at .5" inch long and again my ball striking improved within a few weeks. I don't think I could go any shorter. It is all about control for me.
I know everyone's swing is different, but I think for most taller players we should be playing shorter irons than we think. As tall guys we think we need bigger everything in life and our golf clubs are no different. I remember getting my clubs made to 1.5" inches over standard gave me some satisfaction just knowing I could (I was young). The other point I will make is even the taller pros don't play anything over an .5". Els, Keegan, and Bob Tway play standard length and Bubba Watson and Dustin Johnson use .5". Just my thoughts and experience.