The first attempt

Hmmm...well this is difficult...I have to now choose a song of Rahman and try to discuss it..Well to take the easy route let me take the song "Dil hain Chota sa" from his maiden venture Roja.
Lets first try to visualise the scene in 1992 when Roja hit the screen and of course our ears too. Bollywood music was at an all time low. Nadeem Shravan showed a spark of brilliance with Aashiqui(1990) but then they became too repititive. Like an assembly line they used to compose music with not an iota of innovation be it in rhythm, sound, genre...absolutely nothing. But then the audience had no other way but to rave about these ordinary numbers as masterpieces.
One more thing which became prevalent was blatant copying. Music composers simply lifted tunes of the west (though they humbly call these as inspirations) and fitted it with hindi lyrics. The audience were really treated with some junk piece of music.
Come 1992 and enter a young, humble looking ad jingle composer. Mani Ratnam chose to be different and handed over the mantle of music composing to Rahman rather than his usual choice IllayaRaja. It seems Rahman was on a mission. He wanted to be a revolution and free us from the bondage of some very ordinary music composing. From the entire feel of the music of Roja (be it songs, back ground score etc.) there was something enjoyable happening and we listeners were feeling the joys of independence.
Dil hain Chota Sa: This song was sung by Minmini(nobody heard of her before) and original Tamil lyrics by Vairamuthu which was dubbed in Hindi by P.K.Mishra. From the very beginning of the song we felt that there was a drastic change from the erstwhile ordinary composing. Nobody has ever heard such beautiful usage of bass guitar. It was considered to be an instrument played mostly in the background. Nobody dared to bring it to the fore. Rahman not only did that but made it the soul of the song. The tune was such that it actually expressed the aspiration of a village girl. Nature was just written so beautifully all over the song. You can in fact listen to the discordant sound of a cow at one place which gelled so well with the rhythm. The sound of sticks in the 2nd interlude just highlighted the occupation in a village. The use of flute in the entire song gives a feel of the rustic charm. The beautiful part of this song is the conflict of composition. Nature being highlighted by western style and also the fusion of ethnic sounds was just too much to handle. Listen to the voice of Rahman as well who does a sort of alaap in the 2nd interlude. It was just a spark of his singing ability which has of late flourished like anything.Special mention needs to be given to the voice of Minmini as well. I didnt hear much about her after this song but she just sang it with loads of aplomb.
Beyond doubt it redifined contemporary Indian music. It broke all barriers and made everybody in this world to listen to Indian film music. Music can have so many dimensions: really nobody thought about it. Songs with Indian base, western influence and global appeal...it is like an entire package. Nothing can get better than this.
The journey has just begun and now you will notice unexpected twists and turns very similar to Rahman's style of composing.