There is a definite drop in movie quality when ROTJ came out. What I mean by movie quality, is in Star Wars terms. I love ROTJ, and all Star Wars movies, but when comparing them to one another, that is where I become harsh on them.
ESB is my favorite, with ANH coming in at a close second. Those two movies to me are perfect movies. I can watch them, and I don't wish Lucas would have done anything different to make them better.
ROTS & ROTJ are # 3 & #4, and that is the first drop in quality of a Star Wars movies. I love these movies, but there are certain parts of the movies I wish Lucas would have done different. ROTJ, get rid of those damn Ewoks, and ROTS, can you make Padme die other than just giving up. Also in ROTS, when Anakin turns in about 2 seconds, it isn't very convincing.
Then there are AOTC at # 5, and TPM at #6. They are good Star Wars movies, but far from great. There are many things in these movies I would have done different to make them better, mostly getting away from the kiddy factor.
All in all ROTJ was the first drop in quality in the Star Wars universe, and I don't think Lucas was every able to duplicate ANH & ESB since. Then again, how hard would it be to make 6 perfect/great movies?