To be clear, his take on DADT was quite powerful:
"No matter how I look at this issue, I cannot help but be troubled by the fact that we have in place a policy which forces young men and women to lie about who they are in order to defend their fellow citizens,"But the bigger and more important contribution he made was to return the job of the Chairman to be the senior adviser to the President and SecDef on military matters, even pushing back a bit when his views might not have been those that they sought. Given that Rumsfeld turned the spot into lapdog (or Vulcan mindmeld victim) and Myers and Pace went along with the worst ideas of the Bush crew with nary a peep, Mullen reminded us that the job of the military is to provide the best advice to the President and then implement the orders of the President and SecDef even if they (the military) disagree.
Having the military disagree a bit with the leadership is a natural situation, given their conflicting audiences and imperatives. Advice is just that--making recommendations. These implies two things: that you are not just telling the bosses what they want to hear; AND you know that they don't have to heed the advice. The previous two chairman forgot or ignore their roles and became yes-men. Mullen never seemed to fit that category. He served a SecDef and a President that wanted the best advice--not best defined by guessing what they wanted but informed estimates of the best ways to proceed. And Mullen gave it. Now he is stepping down after a long and distinguished career.
All I can say is: thanks.