Creating a musical theme within a film which is lilting, haunting and memorable, and using it carefully, tastefully and usefully, across your movie is a true and rare art form.
Just realized that the opening instrumental notes of "Main Agar Kahoon" are the same as the piece that Richa Sharma sings to open "Jag Soona Soona Laage" (i.e. the "dil jude bina hi" sequence)
Of course, once you've listened to the Om Shanti Om soundtrack as much as I have, several of these aspects tend to stand out and stare at you. Like if you listen to Dard-E-Disco backwards, you hear the strain, "Shah Rukh's six packs are stick-ons". But maybe that's all the grass.
Background themes with adequate sustaining power in recent years:
(1) The "Ya Maula" sequence in Fanaa: Full marks to Salim-Suleiman for this one.
(2) The "Saathiya" harmony piece by Sonu, in Saathiya. Although, I much prefer its southie counterpart, "Sakhiya Cheliya" by Clinton.
(3) "Ganga, kaahe tu jaaye pardes, chod ke apna des" from Pardes - does anyone remember this? It was really haunting, albeit the opening to that devastatingly annoying "London Dekha Paris Dekha..." song.
(4) The flute theme of Hero - brilliantly reinvented by Nasha.
(5) One word. Karz.
(6) The piano ballad of Kal Ho Naa Ho.
(7) Sukhwinder's "Sitaaron se aage jahaan bhi hai", from Dil Se, gave me chills.
And my absolute favorite, the Bombay theme. A song that made me feel that it was composed in another dimension, altogether. Truly makes you wonder what goes on inside the head of a genius like Rahman, to bring the notes and musical elements together to create such magic. Epic!