It is important to realize that God did not merely make distinctions between Israel and other nations.  He also made distinctions within Israel itself.  The Levites were exalted as priests of the the Lord, guardians of the tent of meeting, and teachers of the Law.  As Israel was God's own people out of all the nations of the earth, so the tribe of Levites was His special possession in Israel.  God took the tribe of the Levites in place of the first-born (males) of each of the other tribes (Num 3:12-3).  

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          “Appoint Aaron and his sons to serve as priests; anyone else who approaches the sanctuary must be put to death" (Num 3:10).

         

Other Levite families (Kohathites, Merarites and Gershonites) were assigned special functions in the care of the articles of worship and the performance of sacrifices.  These assignments were permanent and hereditary.  The Levites were consecrated in Lev 8.

        

Another form of separation within Israel was voluntary.  Any man or woman could take a Nazirite vow for a given period of time (Num 6).  This was a time of special devotion and separation to God.  There were special rules of diet and conduct.  There was no "reward" for this, except for a fellowship offering at the end of the period of devotion.  In other words, God's Presence was the reward.