Imo, he was the first superstar of the sport. The sort of player who transcends a sport and becomes a name and a face to the average Joe/Jolene on the street who has little or no interest in the sport in which they ply their trade - similar to, say, Armstrong in cycling*. And in doing so he basically dragged the game into the professional era.
While his bulk was one of his defining visual characteristics at a time when the sport was changing eras** and the average back was not much bigger than a bloke off the street, he was no average, tits-for-hands bosh merchant like, say, Bastareaud. The guy absolutely oozed skill and class. Even in *that* game against France in '99, his was one of the standout performances - I'll never forget the sight of Garbajosa basically choosing not to bother tackling him as Lomu went over for his second try. Two other tacklers just bounced of him, iirc.
Plus he always came across as a genuinely nice guy.
*In the sense of his fame transcending the sport. No other parallel intended.
**and Lomu was basically the prototype of the modern back, imo.