I love photorealism, and some of my most memorable experiences with painting are from scrutinizing Old Master style works in art museums, but Rothko just scratches an entirely different itch for me.
The first time I saw his stuff, even in a tiny thumbnail on a shitty CRT, the big bright colored shapes, simultaneously rigidly geometric and softly organic hit me somewhere in the back of my monkey brain. I found them soothing and a bit electrifying at the same time.
I was really excited to see some in person at the Guggenheim, framed the way the artist intended. These massive paintings sit at ground level and when you stand in front, they become a doorway into something both desolate and calming.
One thing Rothko forcibly reminds me is how neutered idea of color is in our day to day lives. We're so digital we tend to think a color is just some RGB value boxed in by the limited gamut of an LCD. It's like we listen to music only in 8-bit mono and have forgotten what more there can be.
But if you see an actual painting in the flesh, done by a master who spent a career focusing almost entirely on color, you realize what a powerful thing it can be and how much more your visual senses can be stimulated than they almost ever are.
Looking at a Rothko to me is like hearing a pipe organ in an ancient cathedral. Instead of a flurry of tiny precise notes that make you think "gosh, what a skilled technician", he just lays out one long, monumental chord that you feel in your bones.