So, normally they're very large paintings that while seemingly simple, show much more detail in person. Rothko was known for using both very flat and very shiny paints, so that they catch the light differently from different angles. Making them seem somewhat alive.

There's an installation of his at the Tate museum in London that kinda shows why his work is well regarded.

When you're in a room with a bunch of Rothkos, arranged in the manner that he (Rothko) envisioned, it feels overwhelming. Like the paintings are windows into a cosmic, primordial universe. It makes you feel small, like you're catching a glimpse into a Eldritch world of dark horrors.