It's a very different (and foreign) environment. Job control language, how data is stored... if you come from a typical modern server environment you'd be pretty lost in the mainframe world.
But what I remember most: the two other students were mainframe programmers, and they were just as baffled by my world as I was by theirs. It really was an entirely different computing paradigm, although 30 years later I probably have enough experience to make more connections than I could then.