There are now two generations of Americans who have grown up after the rock revolution of the late 1960s, for whom classical music and the old style Broadway/Hollywood songs are largely marginal. As a result, today's typical American ear is attuned more to rhythm and vocal emotion — the strengths of rock and rap — than to melody and harmony, the strengths of classical music and Golden Age pop. This is true not just of teenagers but of people roughly fifty and under, and has been the most seismic shift in musical sensibility since the advent of ragtime introduced the American ear to syncopation a century ago.
A catchy beat is not just one element, but the sine qua non in most pop today, opening most songs instead of the instrumental prelude of the old days. The increasing popularity of rhythm-centered Third World pop (pointedly called "World Beat") underscores this change in taste. Certainly folks liked a good beat before Elvis, but much of even the most crassly commercial dance music before the 1950s was couched in melody and harmony to a degree largely unknown in today's pop. Our expectations have so shifted that the rock music that critics today call "melodic" would sound like Gregorian chants to members of even the cheesiest little high school dance band in 1930.
Yet what pop has lost in craft it has gained in psychological sophistication, and the focus on vocal emotion is part of this. In the old days, singers made their marks as individuals, no doubt, but, for example, Sinatra's artistry was in being able to suggest a range of emotions within the context of rather homogenous lyrics. Modern pop singers like Alanis Morrisette are freed from these constraints, and the variety and individuality of many modern pop lyrics have made them America's true poetry; indeed, many listeners relate to the lyrics of their favorite rock singers with an intensity our grandparents were more likely to devote to the likes of Robert Frost.
Yet the fact remains that for the typical American of the future, melody and harmony will be as aesthetically marginal as they are to the African musician whose music is based on marvelously complex rhythms, with a vocal line serving largely rhythmic and/or decorative ends (notably, World Beat listeners are little concerned with not understanding most of the lyrics; it's the vocal texture that matters). Lyrics will continue to count, but their intimate linkage to musical line will be of no more concern than individual expression or complex rhythm was to pop listeners sixty years ago. I once attended a screening of a concert video from the mid-1960s in which Sammy Davis, Jr., who occupied the transitional point between the old and the current sensibility, sang Cole Porter's "I've Got You Under My Skin" first "straight", and then without accompaniment, eventually moving into scatting and riffing rhythmically to the merest suggestion of the written vocal line for a good few minutes, in a vein we would today call "performance art". Young hipsters behind me whispered "This is rad!"; a few seconds later I heard an elderly woman in the front row mumble "Enough of this is enough!" She would have been happy to hear Davis simply sing it through once with the orchestra; the hipsters wouldn't have minded Davis walking out and doing only the vocal riffs — and they are the American musical ear of today and tomorrow.