The only mob that i'm aware of that offers worthwhile abilities is a level 42-43 silver elite mob named snarler. And even at that, its only just the innate 100 resistance to all elements, which is nice, but not really an ability. Other than that, i've yet to see any real new abilities that can be obtained from normal or elite mobs.
More info on snarler can be found here: http://forums.worldofwarcraft.com/thread.aspx?fn=wow-hunter&t=54133&p=1&tmp=1#post54133
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