Played on bad pitches with a relatively heavy football and made great defenders look silly, in an era when defenders could absolutely shut out an attack and not give them a sniff. When he performed his Cruyff turn he actually beat players with it, because he did it with pace and at the right time. Nowadays players perform the Cruyff turn, but they don't actually perform it to beat players, but rather just to put the ball on the other foot.
One little story I know of about Cruyff and Koeman together is when Johan was manager of Barcelona and Ronald was his CB. There was an occasion where he dropped Ronald for a match. Now I'm not 100% of the actual words, but Koeman went to Cruyff and asked why he was dropped. Cruyff answered that he was changing the team round. Koeman answered, Boss, you'll drop me once and once only. Cruyff never dropped Koeman again, such was his respect for his main man.