I wrote about this in Charging the Mound and also in a comment on DDD's blog, but Rivera's HR/9 rate is such an outlier, it deserves its own separate mention:

Mariano Rivera was widely praised after he saved his 500th game last week, but one thing I didn't see mentioned in his resume was his place on the active career leaders in HR/9. Rivera has allowed .493 HR/9, while the second player on the list is Brandon Webb at .627, then Derek Lowe at .708. After that, the next 13 players are between .70 and .80. Trevor Hoffman is 17th at .801. Part of the reason for that is few closers qualify for the 1000 IP minimum, but that's also the point. The starters can't touch his peripherals, and the relievers can't hang for more than a couple years. (If you're curious Joe Nathan is at .80 for his career and Jon Papelbon at .68).

In other words, no pitcher in baseball with 1000 innings under his belt is even close, and that includes extreme ground ballers, and another elite closer who played in Petco most of his career. Even the other two elite closers of the last few years are not close. In fact, except for Webb, the gap between Rivera and everyone else in preventing home runs is cavernous.

Contrast that with the home runs per at-bat leaders among active hitters - there are no such outliers, as the list moves up very gradually from Jim Thome, Adam Dunn, Albert Pujols and ARod.

In other words, Rivera is like some freak hitter who hits a long ball every eight nine at-bats and completely stands out on the list. The closest thing to that is Mark McGwire who hit one every 10.6 at-bats, but even he doesn't stand out to the extent Rivera does.

Of course, it's easier to prevent home runs as a closer than as a starter who has to see hitters several times a game, but even the other elite closers don't come close. (I know Jonathan Broxton is close in his short career, but given his park and the small sample, he needs at least two more elite seasons even to merit inclusion in the conversation).