We all know that Manichithathazhu is a classic, and this belief is reinforced after seeing the remakes by P. Vasu and Priyadarshan. To enjoy the original, you have to see it multiple times as there are subtleties, nuances and various clues left all around. Kishore Kumar wrote an article explaining some of the nuances and now here is another one by Ranjith Nair.
Consider this: in ‘Manichithrathazhu’, there is a strong hint that Nakul (Suresh Gopi) is a neglectful husband – sample the scene where Ganga asks Nakul to bed, and he excuses himself citing the excuse that he has work to attend to. In fact, the lovely Varuvanillarumee Vijanamam Ee Vazhiyil number strongly indicates that Nakul might even be impotent ( a fact that is only indicated at in the various remakes by Prabhu’s nuanced performance). Thus there is a strong foundation laid for the possibility that Ganga is neglected, bored out of her wits, and thus particularly vulnerable and empathetic to the legend of Nagavalli. Instead, here we have Shiney Ahuja all the time grinning like an idiot, and happily nuzzling a rather plump Vidya Balan during an incoveniently placed duet.
The music of Manichitrathazhu was very important too, since the the alternate persona was a dancer-singer clearly well-versed with music; she sang and danced at night, after all. The Pazhamthamizh pattu number (conveniently ditched in the remakes, as the timing of the song placement was slightly awry) was not just a song randomly inserted into a scene; it was clearly the the doctor trying to soothe the alternate persona by singing a song in the same raaga as that of the one that the dancer dances to at night (the doctor hears this earlier, and hence he knows the song, so that loophole is closed as well). [Manichithrathazhu: The Unmaking of a Classic]