The Importance of Rhythm
The purpose of the worship team is to lead the congregation in worship, to enable the members of the congregation to individually participate in a time of corporate worship, typically focused around the singing of songs.
There are two key elements required for the People to fully participate in the songs: Rhythm/beat and Melody. Neither element is sufficient in and of itself, and neither is more important than the other. The melody sits within the framework of rhythm, and the rhythm is the structure against which the melody works.
In order to act as an enabler, rhythm must be distinct/discernable, steady, constant, and predictable. Note that each of the facets has a corresponding hindrance.
The members of the congregation must be able to clearly sense the beat of a song so that they may follow along. If there are two many rhythmic sounds, or competing rhythmic events, then the congregation will be unable to determine which beat to follow.
Not everyone has a good internal sense of rhythm, so it is vital that the beat always be present. This doesn't mean that every quarter-beat, half-beat, etc. must be audible; rather, it means that the beat be regularly heard so the people don't individually drift between audible beats.
While the band is playing and there is an expectation for the people to follow, the tempo, that is the rate at which the beat flows, must not vary. The team shouldn't speed up and slow down as the song progresses. This directive is not intended to completely forbid all tempo changes, ever, but changing tempo mid-song should be very rare.
When we sing and play together we follow a leader; however, this doesn't mean that we don't sing or play each note/syllable until we first hear the leader play that note. Each participant (including the members of the congregation) is individually predicting when the next beat will occur and then singing/playing their part at this predicted time. Based upon whether their note/syllable was on the beat or not, the individual adjusts their internal tempo and predicts the next note to sing/play. If the beat is not predictable then the people become confused and discouraged and they back off. If the people can't predict the notes, they cannot participate.
As long as the participants can predict the elements of a change, they are able to embrace the change and completely participate in it. If the arrangement of a song contains a tempo change, then as long as it is played in a manner that allows the people to anticipate the elements of the change, it will go well.
The most common occurrence of tempo change is the ritard that sometimes is used to end a song. A worship team needs to perform ritards in a consistent way, such that the congregation can follow. Consistent, predictable ritards are those that typically move the tempo from every beat to every second beat (i.e., half-time) or every fourth beat (i.e., quarter-time). While each and every song doesn't have to have its ritard played identically, it is important that the team always play a particular song's ritard the same way (i.e., this is part of the congregation learning the song).
Worship teams commonly exhibit a couple of problems with rhythm and tempo: one or more musicians not playing staying synchronized, and playing/performing portions of songs in a tempoless fashion.
Not every member of the worship team is able to keep an even tempo; in fact, in my experience, when left to themselves, most worship team members are not able to keep a constant beat. To compensate for this, choose one band member who does have the ability and have every other band member synchronize to him/her. It works best if this person plays a rhythm-based instrument like the drums, piano, bass guitar, or acoustic guitar.
Never lose sight of the fact that this dependence upon one another is a good thing. None of us is created perfectly, and it is God's intention that we utilize each one's gifts for the benefit of all. That said, it does limit an individual's expression when the rest of the team is counting on them to keep everyone honest (from a tempo perspective).
It is incredibly frustrating for the congregation when the worship team plays a song which has no definable regular rhythm. The people in the congregation are unable to participate in the song, and the song breaks down into performance instead of worship leading.
Even the most experienced musicians exhibit this behaviour, and so it is not directly related to inexperience leading worship. The most common manifestations of this problem I have encountered are: