I have noticed great differences since leaving (truly mentally out for 9 months and have not been to a meeting for 4-5 months). The difference has not been so much in my mental thought processes but more of a weight lifted; cognitive dissonance was a heavy burden to bear and it is only its absence which has made me realise this.
As for independent thinking etc., I can relate very much to what Steve Hassan states but to a more limited degree than many Witnesses I have encountered. Even though I was born into the Witnesses I have always had an independent mind and questioned things even as a small child: family members would half jokingly call me an apostate whenever I would object to a line of reasoning contained in one of the society's articles (no matter how watertight my logic was nor how specious the Watchtower reasoning). I suppose that I was a "good Witness" in that I would always place these objections aside in the end due to the mindset that told me "most of the teachings are right so wait on Jehovah."
It was when my independent thinking met up with articles by "worldly" theologians that the cracks really started to appear. I found their eloquence and deep ponderings to be far more stimulating than any Watchtower articles I ever read. It was not long after that I began to question the organisation as a whole.
Of course, another difference is that I can now follow through on my thoughts and come to my own conclusions without being constrained by Watch Tower dogma in any way at all. The independent thinking was always there, and it earned me some horrified and fearful lookes at times, but I always kept it in check to a large degree. Now it just feels wonderful to be able to use my mind fully even if I cannot speak about my thoughts to most of my family who are still in.