Just for starters:
"O Come, O Come Emmanuel" is often sung incorrectly by too many parish music leaders who have no training in reading music. What happens is that they put a long rest after Emmanuel  in the line "Rejoice, rejoice, Emmanuel shall come to thee O Israel." I hope people remember not to do this in the approaching Advent season.
Re recessionals, they should be fairly simple tunes which can be sung by the congregation and in the case of recessionals, as they are leaving the church (a recessional is after all just that - a recessional).
Far too often the "recessional" turns out to be a six or even seven stanza hymn which goes on and on and congregation look at their watches after the fourth stanza wondering how long this is going to go on for - completely spoils it.
(I will discuss my pet hate - the song "Here I am to worship" in part 2).