"Layers" isn't really relevant and has never been what's made songs good, and dynamics not really appropriate for a Kara stan to bring up (SweeTune productions are monotone walls of sound - in a good way of course).
Going to have to disagree. While it's true that layers aren't needed to make a song good, if you only have a few, they better be doing something good with their part. Since the parts on T-ara songs generally hardly go anywhere, more would certainly help. Likewise, if the layers are arranged well, they can be a huge part of what makes a great song great.
Dynamics also doesn't always mean a range of mood or intensity, but a great deal of them lies in how the instruments play off each other. If you were to be playing in a band, how connected would you feel with the other musicians, working together with them and feeding off what they are doing, offering support at times, jumping in and out of spotlight as it is appropriate to the flow of the song, etc.
Take for instance, Rock U. It's seems fairly simple if you're not paying attention, but listen properly and notice what all is there. To do it live you would need drums, two electric guitars, acoustic guitar, two sets of trumpets, keyboard (switching modes on the fly), bass, and backup singers beyond the girls' parts. But it's not just those layers, it's what they are all doing and how they work with each other. It's fantastically woven together, and there are a lot of dynamics in that interaction as well as the flow of the song.
Beyond that, just looking at each instrument, a few of them are doing very interesting things, particularly the bass. The vocal melody is also quite nice, mixing light loveliness with playful energy, covering a nice range of scale with a decent amount of harmonies. Go from that to something like Roly Poly and... well even understanding that Roly Poly is a club song made for different purposes, it's flat and cheap to my ears. It's an itch faaaar better scratched by YG offerings.
