Selecting just the right key for a song can be tricky, but we have to try. As worship leaders, our chief responsibility is to employ musical elements in order to engage the congregation, helping them connect with the Holy Spirit and worship God. If the congregation can't physically sing a song, it's a non-starter.
Let's start with how not to choose keys, and take it from there.
First of all, there is no single key that a song belongs in. Chances are, we've come across new music in the context of a concert or album, so the key in which we'd first experienced the song was best suited for a large performance venue or listening experience. Yes, I've had plenty of powerful worship experiences in these settings, but they aren't necessarily what we're working with on a Sunday morning. Interestingly, the more I've experienced songs in different settings, the more I've learned that the songwriters themselves change the songs' keys all the time, based on factors like venue or even time of day.
As you choose keys for your worship service's songs, if the first question is "can I crush this?," you might be on the wrong track. There's great value in a worship leader being confident and comfortable when leading a song, but there's greater value in the congregation singing along. The "I Sound Awesome" test is great for albums and concerts, but not so much for Sunday mornings.
If the key that best suits congregational singability is Bb, don't worry about the band knowing how to play in that key. That's what rehearsal is for. That's what strong instrumentalists are for. I understand that moving the key too far up or down from the original can negatively effect guitar voicings and riffs, so we should be mindful of that but trust our instrumentalists to work around it.
Now, here are some things to strive for as we choose keys.
To quote, well, myself, "It's nearly impossible to know what your congregation's vocal boundaries are, but in general, try to keep the low notes above low C and the high notes below high D. Men, women, boys, and girls all have different comfortable vocal ranges, but this is at least a good place to start (Songwriting for the Church)." The more we push those boundaries, the more obstacles we create for a worshiper to follow what we're leading, and that sort of goes against everything we set out to do as worship leaders.
To be fair, there's still great value in setting songs in the higher parts of common vocal range. The higher notes can offer quite the positive effect, as they require more breath support to be produced, thereby requiring more whole-body involvement and energy from the singer. Also, the larger and louder the venue, the higher people tend to voluntarily sing. But, we have to acknowledge the delicate balance, here. If the song lives in the higher register, or sustains a higher note for too long, or pushes trouble vowels ("eee") too high, voices will strain and fatigue, therefore discouraging participation by singing.
Transitions between songs can make or break an atmosphere of connectedness with God. Any drop of continuity will allow distraction to enter in, falling in the face of the worship-focused environment we hope to create. So, if shifting the key a step or so will make for a smoother hand-off from song to song, go for it! Otherwise, you can always use this handy Transition Chart.
Over the last few years, it's been popular to write worship songs with huge vocal ranges and octave jumps. Just this past Sunday, I led two songs each with a 13-note range. While the congregation can always drop or jump octaves to sing in whatever range is most comfortable, I'd be curious to see how many actually do that. Even less likely is that a worshiper would find a harmony part that fits his or her comfortable range.
In the end, choosing the right key is just as much an algorithm as it is a feeling. There's an abundance of factors to keep in mind, some personal and some corporate. There's no such thing as the universally-perfect key for any song, but we worship leaders can still seek to serve the congregation by including this process as we plan our sets.