The thing about GDM was that, for a pace bowler, he had a great subtlety to his bowling that lots didn't. Because of his action, he could adapt and read a batsman's movements very late - an example is how he knocked over Kallis in the semi of the WC - Kallis moved late, and as McGrath said - he saw him move about a metre before he let the ball go, and adjusted with a perfect off stump yorker. Also, for a bloke who, by this year, had lost a fair bit of his pace, he still tied batsmen down on small grounds in the WC and got them out. Look at how he hit Pietersen and broke his rib in the CB series - he said to KP the ball before that he was starting to read him.
The guy was/ is a freak. The fact that he made something which is so difficult look so easy is a credit to him, but something which in some ways detracts from his mystique - he wasn't all fire and brimstone like a Lillee, didn't have the silky smooth action of Hadlee or the explosive pace of a a Marshall. With Lillee, people still remember the bristling moustache and the attitude, they still remember Marshall's awesome pace, Akram's volatile swing. It's a lot less exciting to look back and say "I remember McGrath's wonderful line and length", even though it's probably a greater attribute than even any of the other's I've mentioned.