The key is being able to bowl it consistently, along with a variety of slower balls. Steyn hovers around the 140km/h mark, with the odd bouncer reaching 150km/h. He just doesn't strike me as the sort of player who can place 4 out of 6 in the blockhole at 150km/h like Bond, Lee, Tait and Malinga can do.
But I guess he's probably the closest we have at the moment - I just think that they shouldn't chuck him into the team and expect him to perform only to drop him after 2 matches. He needs to be trained to consistently bowl fast. Also our twenty20 specialist batsmen need to be trained to prolong their style of batting over a 50-over period. Imagine the force we could be if Bosman, JP, Morkel etc could achieve this.
Personally I'd love to see Morkel in the team in place of Prince. I love how aggressive he can be.