Hoffman did have a pretty insane reliever peak:
1996-2000: http://www.fangraphs.com/leaders.aspx?pos=all&stats=rel&lg=all&qual=y&type=8&season=2000&month=0&season1=1996&ind=0&team=0&rost=0&age=0&filter=&players=0&sort=19,d
I guess his case is that he surrounded an elite five year peak with longevity. But on the career scale, he's still not even comparable to Mo. He's closer to frigging Papelbon and K-Rod than he is to Rivera.
At the end of the day the difference between Trevor Hoffman and guys like Jonathan Papelbon, Joe Nathan, or K-Rod is that Hoffman was able to tack about five (5) extra 1 WAR seasons onto his career. That's a pretty awful HOF case.
Of course, the voters also care about 600 Saves and we absolutely do not because we are smart and they are dumb. But it is what it is (dumb).