For me it's the guitar solos and also the driving rhythms. At the time in the 70s there wasn't and had not been anything called metal and although Led Zeppelin and Black Sabbath were both big. The lyrics are also non-trivial. A lot of the music is what used to be called growers, i.e. they grow on you the more you listen to them. DSOTM, Animals, Wish You Were Here and The Wall, Meddle.