Adding details such as first communion dates and religion
I have found in my research that I often come across first communion dates in records, and, while I can add this as a custom event, it's such a common thing, I feel it would be helpful to add to the list of regular events. If nothing else, these dates help members of the church know whether a child has lived to be over 8 years old. Sometimes this is the only indication we have. I am able to add such detail in Ancestry, but not to Family Search except by a custom event.
I also think it would be helpful to be able to add religion details to records. This could be added as a note, but being able to pinpoint religion on the main details page would help point family history collaborators in the right direction. For example, knowing that my German ancestors were Catholic helped me know where to begin tracking down lost branches.
It would also be great if I could filter by religion when searching for available resources. I often find myself sifting through piles of Lutheran resources when I need Catholic. :-)