What does it mean to "fear God" when so many angelic messages start with "do not be afraid"?

The angel in Revelation 14 who shouts "fear God" is unique in the Bible, because no other angel is recorded with this message. In fact, the phrase "fear God" is not all that common in the Bible. However, plenty of angels come with the message "do not be afraid", and we turn to some of these examples to stimulate thought and conversation. We look at Hagar (Gen 21), Gideon (Judges 6), Balaam (Numbers 22), Elijah (2 Kings 1), and Mary and Zechariah (Luke 1).