I rented the Blu-ray version of FOTR and A/B compared it with my DVD version in my Oppo 981 with the BD in my Oppo BDP-83.
All I really noticed was an improvement in some detail. Though there were better black levels and colour pallet in the BD version I attribute that to the player which exhibit’s the same improved performance when comparing identical standard DVDs. Note I was watching on a 134” diagonal so the improved detail may not be noticeable as the screen size gets smaller.
The difference in audio was a mixed bag. Overall I think it was better, however I had grown so accustomed to hearing certain things (I use FOTR as a calibration check, and demo) the differences threw me off a big. Mostly changes in the LFE making them seem less pronounced in the BD version though the overall presentation seemed to have more clarity. Note that I could make all the comparisons I wanted because I always use the extended version not the theatrical so some of my demo scenes weren’t there.