This is taken from a discussion on the worship central forum on Christmas carols:
A simple definition of worship that I often use is “praising God for who He is and what He’s done”, therefore a ‘worship song’ should praise God for who He is and/or what He’s done.
“O come all ye faithful” works because it is praises God for who He is (O come let us adore Hime, Christ the Lord) and “Once in Royal David’s City” works because it praises what He’s done by retelling the story (He came down to earth from heaven, Who is God and Lord of all…).
Some are less clear, you could argue that carols like “While Shepherds watched their flocks by night” are not worship songs as they are not addressed to God however the converse argument could be made as they are clearly telling of what He’s done. Some Carols have some dodgy history in them but are still strong worship songs, e.g. “In the bleak midwinter” (it really wasn’t snowing!), the last verse of which is one of the most powerful challenges of any Christmas carol.
Some carols, like “Good king Wenceslas” could more accurately be described as folk songs, telling a nice story with a fuzzy ‘be good to the poor’ moral at the end. I would avoid using these as worship songs..
Of course some are just bad songs (I would include “Away in a manger” in this category!), regardless of their historical or theological accuracy a bad song is a bad song!
In the end a worship song should point people towards God and enable them to praise Him for who He is and what He’s done.