The standard parameters would be musical and lyrical quality. Within musical quality, it is possible (although not always profitable) to separately evaluate melody and percussive elements.
Then there is the song itself considered as a whole, combining the separate elements. This is equal to, but usually much greater than the mere sum of the elements. Consider this song from Door Gagan ki Chhaon Me (1964)
'Aa Chal ke Tujhe', Kishore Kumar
In terms of both melody and lyrical structure, this song is commonly regarded as a 'classic', having withstood the greatest test of all, which is the test of time. More interesting for our purposes, the song is, on both parameters, quite integrated with the notions it describes: a dream of a better world, rendered against a reality that falls decidedly short. So the lyrics are straightforward, clear and emotive, the melody line that accompanies it is simply that -- a melody line that offsets significant ideas of the world which are expressed, in a specific manner, within the lyrics.
The reason I am doing this impromptu (and shorthand) analysis is that I am currently somewhat bemused by the reaction to a contemporary song called 'Kolaveri Di', by an audience that cut its musical ears on songs such as the 1964 classic you just heard. I do understand the rebellion of youth, the need to move on, and updated modes of expression, but it is somewhat alarming when these otherwise reasonable impulses seem also concomitant with a depressing lack of ability to distinguish between good, bad and indifferent music.
About 'Kolaveri Di'. These are the lyrics:
pa pa paan pa pa paan pa pa paa pa pa paan
Lyrics, then, are clearly not the most meaningful element of this song.
If the lyrics are this asinine, one thinks, then the musical scores must be marvellous, rendered by a truly unusual, rich, complex voice. Sometimes voices and musical scores are so breathtaking that it is possible to miss some obvious embarrassments in the lyrical sphere. An example of this is songs from the 'Buena Vista Social Club', based in Cuba. In these songs, the beauty of the language of the lyrics, combined with truly attractive melody and percussion lines, hide the cliched banality of the meanings of the lyrics. Let me demonstrate:
Buena Vista Social Club [live], Chan-chan
Look up the lyrics for this song. They are not particularly profound, or even interesting. But the music is wonderful, and the singer has a range that belies his eighty-odd years. Now, here is 'Kolaveri D'
That's just monotonous: really pointless lyrics, whiny tonality, perhaps mildly attractive at best. Why then, the hype? Well, Dhanush is married to 'Rajini Sir's' daughter Aishwarya, and that's always a draw in icon-addled India. That's also a very solid reason for the media machine to do the equivalent of word-of-mouth. After all, we all know that Rajinikant cannot -- and should not -- be crossed.
Jokes aside, there's plenty of reasons that a media system that thrives on stardom has an investment in creating and retaining stars. And even more reasons why Rajinikant should be kept happy :)
What I am suggesting is that the popularity of this song has little to do with the qualities of the song itself, and more to do with externals, principal among which are (a) the star-connection of the protagonists (Dhanush and Aishwarya), and (b) qualities endemic to the technology of the disseminating medium itself.
This is perhaps one of the first examples of a combination of the classic eastern tradition of rumour, and a sophisticated understanding of the putting out, and retrieval of information on the internet.
Search heuristics, engines, and niche marketing using social networks are clearly the things of the future. I think that the strange popularity of 'Kolaveri Di' gives us a picture of that media future.