I watched a Taylor documentary a few weeks ago, and it what struck me about her music was not so much that she writes it all herself, but that she manages to convince her followers that the same three chords (almost exclusively F, C, and G, which can effectively be played by moving a single finger from string to string all on the third fret: the Hot Crossed Buns of guitar wizardry) combined with a different catchphrase equal a dozen distinct and individual compositions. For a pianist, it’s basically the C, Dm, Em, F (all white key triads) of song creation. How many ways can we repurpose Heart and Soul?
Not sure it's fair to criticize TS based solely on her repeated use of the same 3 - 4 chords, as I'd estimate over 90% of popular music (good and bad), in some sense, relies on the same relative chord structure which lies at the foundation of western music.
The central "key" that a song is written in is called the "root" chord. The two chords that naturally "fit, or sound good" in that key are called the 4th and the 5th chords (which are 4 and 5 steps away form the root note on the piano). There is also a "relative minor" belonging to each key, which we would find on the 6th note.
So, in the key of C - the F chord is the 4th and the G chord is the 5th, with Am being the relative minor.
But if you start in the key of G - the C chord is the 4th and the D chord is the 5th, and Em being the minor.
In that sense, you can play almost any song in any key as long as you know the root and it's relative 4ths and 5ths.
Heart and Soul is a good example of following a Root - Minor - 4th - 5th progression (C - Am - F - G), which is shared by other popular 50's Do-Wop songs like "Dream and Little Dream of Me" by the Everly Brothers, Earth Angel, Blue Moon, Stand By Me, etc.
The 12 Bar blues progression is another good example of how the exact same chord progression can be crafted into a myriad of different songs, each one unique and interesting in itself, but based on the same overall pattern.
It most often looks something like this - Root (4 bars) - 4th (2 Bars) - Root (2 Bars) - 5th (1 Bar) - 4th (1 Bar) - Root (2 Bars)
And can be found is such Classics as "Rock Around the Clock" by Bill Hailey and the Comets, "Tutti Frutti" by Little Richard, "Hound Dog" by Elvis and "Travelling Band" by CCR. The Beatles themselves recorded over 25 different songs (some originals, some covers) using this 12-Bar Blues pattern or a slight variation thereof.
Hopefully, all the non-musicians out there can forgive me for delving a little into musical theory. I brought it up to illustrate that it is not necessarily simplicity or using familiar well-worn chord progressions that makes a song boring or superficial. At the same time, music that is more complex using uncommon or unrelated chords and unique patterns is not necessarily better or more interesting.
I like to think of popular music kind of like a chessboard, where there can be an infinite amount variety within a finite space. Where a chessboard is confined to 64 squares, no two chess games are played out exactly the same. And using a traditional 3 chord (root, 4th, 5th) in different combinations, there can be amost endless variety within as evidenced by how many widely different popular songs have been made using only this formula.
For me, a really good song consists of a perfect marriage of chord progression, melody line and lyrical content, and although it's rare that a song contains all three in equal degrees, it does seem that certain songs that "stand the test of time" are well-represented in these three aspects. From what little of her music I've heard, TS seems rather mediocre in all three areas.
So, I agree as others have said, what differentiates the music of TS from other older classic pop music is, for want of a better term, the lack of heart and soul. Taylor Swift herself (and by extension her music) is a "product" of the music industry, made for mass consumption, like a plastic piece of fruit wrapped in cellophane, that looks fresh and shiny but whose beauty is mostly fake and superficial.
If popular music is a "reflection" of the times people are living in, then the music of Taylor Swift is perfect mirror of life on 21st century planet earth.
Not only that - I mean, there's many songs with 3 chords that are awesome, so it's not just complexity that is missing. In fact, the Cs advised once to keep it "simple and beautiful" when making music together (though in a different context).
I think what's missing is artists who are in touch with their souls, and with reality. This is the precondition to writing lyrics (and music!) that can actually touch those with half a soul still firing, expressing a deeper truth. The rest is "just" talent, hard work etc., but without the foundation, what can you do?
Yes, this is a better version of what I was trying to say!